#include <iostream>

int main() {
	int x;
	std::cin >> x;

	int ret = -1;
	int l = 1, r = x;
	while (l<=r) {
		int m = (l+r)>>1;
		std::cout << "? " << m << "\n";

		int resp;
		std::cin >> resp;

		if (resp==-1) {
			return 0;
		}
		if (resp==0) {
			l = m+1;
		}
		if (resp==1) {
			ret = m;
			r = m-1;
		}
	}

	std::cout << "! " << ret << "\n";
}