#include <bits/stdc++.h>

#define ll long long
using namespace std;

const int N = 2e2 + 5, M = 15;

void tc() {
    ll n;
    cin >> n;
    if (n == 1) {
        cout << 1 << '\n';
        return;
    }
    if (__builtin_popcountll(n - 1) == 1 or __builtin_popcountll(n - 3) == 1) {
        ll pos = n / 2;
        if (pos % 2 == 0)
            pos++;
        cout << pos << '\n';
    } else cout << "impossible\n";
}

signed main() {
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int t = 1;

//    cin >> t;

    while (t--) {
        tc();
    }

    return 0;
}