library SafeMath {
 
     /**
     * @dev Multiplies two numbers, throws on overflow.
     */
     function mul(uint256 a, uint256 b) internal pure returns (uint256) {
         if (a == 0) {
             return 0;
         }
         uint256 c = a * b;
         assert(c / a == b);
         return c;
     }
 
     /**
     * @dev Integer division of two numbers, truncating the quotient.
     */
     function div(uint256 a, uint256 b) internal pure returns (uint256) {
         // assert(b > 0); // Solidity automatically throws when dividing by 0
         uint256 c = a / b;
         // assert(a == b * c + a % b); // There is no case in which this doesn't hold
         return c;
     }
 
     /**
     * @dev Substracts 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;
     }
 
     /**
     * @dev Adds two numbers, throws on overflow.
     */
     function add(uint256 a, uint256 b) internal pure returns (uint256) {
         uint256 c = a + b;
         assert(c >= a);
         return c;
     }
 }
/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0) {
        return 0;
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c;
}

/**
 * @dev Multiplies two numbers, throws on overflow.
 */