upgrade openzepplin contracts version and match our contracts to the …

…latest versions

contracts/access/Governed.sol

@@ -1,4 +1,4 @@
-pragma solidity >=0.6.0;
+pragma solidity >=0.8.0;
 
 
 /**

contracts/access/Ownable.sol

@@ -1,4 +1,4 @@
-pragma solidity >=0.6.0;
+pragma solidity >=0.8.0;
 
 /**
  * @title Ownable

contracts/access/ReentrancyGuardOptimized.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity >=0.6.0 <0.8.0;
+pragma solidity >=0.8.0;
 
 /**
  * @dev Contract module that helps prevent reentrant calls to a function.

contracts/bepro/INetworkV2.sol

@@ -1,4 +1,4 @@
-pragma solidity >=0.6.0;
+pragma solidity >=0.8.0;
 pragma abicoder v2;
 
 import "./token/ERC721/BountyToken.sol";

contracts/bepro/NetworkRegistry.sol

@@ -1,9 +1,9 @@
-pragma solidity >=0.6.0 <=8.0.0;
+pragma solidity >=0.8.0;
 pragma experimental ABIEncoderV2;
 
-import "@openzeppelin/contracts/math/SafeMath.sol";
+import "@openzeppelin/contracts/utils/math/Math.sol";
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
-import "@openzeppelin/contracts/utils/EnumerableSet.sol";
+import "@openzeppelin/contracts/utils/structs/EnumerableSet.sol";
 import "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
 
 import "./INetworkV2.sol";
@@ -18,7 +18,7 @@ import "./token/ERC721/BountyToken.sol";
  */
 contract NetworkRegistry is ReentrancyGuard, Governed {
 
-    using SafeMath for uint256;
+    using Math for uint256;
     using EnumerableSet for EnumerableSet.AddressSet;
 
     uint256 constant MAX_PERCENT = 100000000;
@@ -100,8 +100,8 @@ contract NetworkRegistry is ReentrancyGuard, Governed {
         require(_amount > 0, "L0");
         require(erc20.transferFrom(msg.sender, address(this), _amount), "L1");
 
-        lockedTokensOfAddress[msg.sender] = lockedTokensOfAddress[msg.sender].add(_amount);
-        totalLockedAmount = totalLockedAmount.add(_amount);
+        lockedTokensOfAddress[msg.sender] = lockedTokensOfAddress[msg.sender] + _amount;
+        totalLockedAmount = totalLockedAmount + _amount;
 
         emit UserLockedAmountChanged(msg.sender, lockedTokensOfAddress[msg.sender]);
     }
@@ -132,7 +132,9 @@ contract NetworkRegistry is ReentrancyGuard, Governed {
 
         require(erc20.transfer(msg.sender, lockedTokensOfAddress[msg.sender]), "UL3");
 
-        totalLockedAmount = totalLockedAmount.sub(lockedTokensOfAddress[msg.sender]);
+        (, uint256 newTotalLockedAmount) = totalLockedAmount.trySub(lockedTokensOfAddress[msg.sender]);
+
+        totalLockedAmount = newTotalLockedAmount;
         lockedTokensOfAddress[msg.sender] = 0;
 
         emit UserLockedAmountChanged(msg.sender, lockedTokensOfAddress[msg.sender]);
@@ -152,22 +154,28 @@ contract NetworkRegistry is ReentrancyGuard, Governed {
      */
     function registerNetwork(address networkAddress) nonReentrant external {
         INetworkV2 network = INetworkV2(networkAddress);
-        uint256 fee = (lockAmountForNetworkCreation.mul(networkCreationFeePercentage)).div(MAX_PERCENT);
+        uint256 fee = lockAmountForNetworkCreation.mulDiv(networkCreationFeePercentage, MAX_PERCENT);
 
         require(networkOfAddress[msg.sender] == address(0), "R0");
         require(lockedTokensOfAddress[msg.sender] >= lockAmountForNetworkCreation, "R1");
         require(network._governor() == msg.sender, "R2");
         require(address(network.registry()) == address(this), "R4");
 
         if (treasury != address(0)) {
+            (, uint256 newTotalLockedAmount) = totalLockedAmount.trySub(fee);
+            totalLockedAmount = newTotalLockedAmount;
+
             require(erc20.transfer(treasury, fee), "R3");
-            totalLockedAmount = totalLockedAmount.sub(fee);
         }
 
         networksArray.push(network);
         openNetworks[networkAddress] = true;
         networkOfAddress[msg.sender] = networkAddress;
-        lockedTokensOfAddress[msg.sender] = lockedTokensOfAddress[msg.sender].sub(fee);
+
+        (, uint256 newUserLockedAmount) = lockedTokensOfAddress[msg.sender].trySub(fee);
+
+        lockedTokensOfAddress[msg.sender] = newUserLockedAmount;
+
         emit NetworkRegistered(networkAddress, msg.sender, networksArray.length - 1);
     }
 
@@ -202,7 +210,8 @@ contract NetworkRegistry is ReentrancyGuard, Governed {
         EnumerableSet.AddressSet storage pointer = transactional ? _transactionalTokens : _rewardTokens;
         uint256 len = _erc20Addresses.length;
 
-        require(len.add(pointer.length()) <= MAX_ALLOWED_TOKENS_LEN, "AT0");
+        (, uint256 tokensLength) = len.tryAdd(pointer.length());
+        require(tokensLength <= MAX_ALLOWED_TOKENS_LEN, "AT0");
 
         for (uint256 z = 0; z < len; z++) {
             require(pointer.add(_erc20Addresses[z]) == true, "AT1");