library SafeMath {

/**
* @dev Multiplies two numbers, reverts on overflow.
*/
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
if (a == 0) {
return 0;
}

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

return c;
}
function div(uint256 a, uint256 b) internal pure returns (uint256) {
require(b > 0);
uint256 c = a / b;

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

return c;
}

function add(uint256 a, uint256 b) internal pure returns (uint256) {
uint256 c = a + b;
require(c >= a);

return c;
}
function mod(uint256 a, uint256 b) internal pure returns (uint256) {
require(b != 0);
return a % b;
}
}
/**
 * @title SafeMath
 * @dev Math operations with safety checks that revert on error
 */
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
if (a == 0) {
return 0;
}

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

return c;
}
/**
 * @dev Multiplies two numbers, reverts on overflow.
 */