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