#include <bits/stdc++.h>
using namespace std;

using i64 = long long;

int main() {
    cin.tie(nullptr)->sync_with_stdio(false);
    
    auto solve = [&]() {
        auto check = [&](i64 x) {
            return x == (x & -x);
        };

        i64 n;
        cin >> n;

        if (check(n - 1)) {
            cout << (n + 1) / 2 << '\n';
            return;
        }

        for (i64 t = 2; t + 1 < n; t *= 2) {
            if (n - t - 1 > 0 && check(n - t - 1)) {
                cout << t + 1 << '\n';
                return;
            }
        }

        cout << "impossible\n";
    };

    solve();

    return 0;
}