SafeMath.sol

// SPDX-License-Identifier: MIT
pragma solidity >=0.7.0 <0.8.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
 
library SafeMath {
    function safeAdd(uint a, uint b) internal pure returns (uint c) {
        c = a + b;
        require(c >= a);
    }
    function safeSub(uint a, uint b) internal pure returns (uint c) {
        require(b <= a); c = a - b;
    }
    function safeMul(uint a, uint b) internal pure returns (uint c) {
        c = a * b; require(a == 0 || c / a == b); 
    }
    function safeDiv(uint a, uint b) internal pure returns (uint c) {
        require(b > 0);
        c = a / b;
    }
}