library SafeMath {
/**
* @dev Multiplies two unsigned integers, reverts on overflow.
*/
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
// Gas optimization: this is cheaper than requiring 'a' not being zero, but the
// benefit is lost if 'b' is also tested.
// See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
if (a == 0) {
return 0;
}

uint256 c = a * b;
require(c / a == b);

return c;
}

/**
* @dev Integer division of two unsigned integers truncating the quotient, reverts on division by zero.
*/
function div(uint256 a, uint256 b) internal pure returns (uint256) {
// Solidity only automatically asserts when dividing by 0
require(b > 0);
uint256 c = a / b;
// assert(a == b * c + a % b); // There is no case in which this doesn't hold

return c;
}

/**
* @dev Subtracts two unsigned integers, reverts on overflow (i.e. if subtrahend is greater than minuend).
*/
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
require(b <= a);
uint256 c = a - b;

return c;
}

/**
* @dev Adds two unsigned integers, reverts on overflow.
*/
function add(uint256 a, uint256 b) internal pure returns (uint256) {
uint256 c = a + b;
require(c >= a);

return c;
}

/**
* @dev Divides two unsigned integers and returns the remainder (unsigned integer modulo),
* reverts when dividing by zero.
*/
function mod(uint256 a, uint256 b) internal pure returns (uint256) {
require(b != 0);
return a % b;
}
}
/**
 * @title SafeMath
 * @dev Unsigned math operations with safety checks that revert on error
 */
function mod(uint256 a, uint256 b) internal pure returns (uint256) {
require(b != 0);
return a % b;
}

/**
 * @dev Divides two unsigned integers and returns the remainder (unsigned integer modulo),
 * reverts when dividing by zero.
 */