#include <algorithm>
#include <vector>
#include <iostream>
#include <bit>

void solve() {
    unsigned long long int n;
    std::cin >> n;
    if (n <= 3) {
        std::cout << "1\n";
        return;
    }
    // 判断 x 是否可以拆成两个二进制数位
    auto judge = [&](unsigned long long int x) ->bool {
        return std::popcount(x) <= 2;
    };
    auto v = n - 1;
    if (judge(v)) {
        if (std::popcount(v) == 1) {
            std::cout << (v >> 1) + 1 << "\n";
        } else {
            std::cout << (v & (-v)) + 1 << "\n";
        }
    } else {
        std::cout << "impossible\n";
    }
}

int main() {
    solve();
    return 0;
}