pragma solidity ^0.5.1;

library SafeMath {
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "The second parameter can not be greater than first one.");
        uint256 c = a - b;
        return c;
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "The first parametert can not be greater than the sum of two parameters");
        return c;
    }

    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b != 0, "The second parameter can not be zero");
        return a % b;
    }
}