library SafeMath {
    // Multiplies two numbers, throws on overflow./
    function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
        if (a == 0) return 0;
        c = a * b;
        assert(c / a == b);
        return c;
    }
    // Integer division of two numbers, truncating the quotient.
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return a / b;
    }
    // Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;
    }
    // Adds two numbers, throws on overflow.
    function add(uint256 a, uint256 b) internal pure returns (uint256 c) {
        c = a + b;
        assert(c >= a);
        return c;
    }
}
/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error.
 */
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;
    }
// Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).