from sys import stdin, stdout

input = lambda: stdin.readline().rstrip()
write = stdout.write

def main():
    min_ = 1
    max_ = 1000000000
    mid = max_ // 2
    while 1:
        print('?', mid)
        stdout.flush()
        res = int(input())
        if res:
            min_ = mid
        else:
            max_ = mid - 1

        n = max_ - min_
        if n < 2:
            break
        else:
            mid = min_ + n // 2

    print(min_)
    stdout.flush()

main()