library SafeMath {

  // We use `pure` bbecause it promises that the value for the function depends ONLY
  // on the function arguments
    function mul(uint256 a, uint256 b) internal pure  returns (uint256) {
        uint256 c = a * b;
        require(a == 0 || c / a == b);
        return c;
    }

    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a / b;
        return c;
    }

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a);
        return a - b;
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a);
        return c;
    }
}
function mul(uint256 a, uint256 b) internal pure  returns (uint256) {
    uint256 c = a * b;
    require(a == 0 || c / a == b);
    return c;
}

// We use `pure` bbecause it promises that the value for the function depends ONLY
// on the function arguments