#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) < 4) {
        int cnt = 0;
        int pos;
        ll temp = n;
        while (temp > 0) {
            if (temp & 1)
                pos = cnt;
            cnt++;
            temp /= 2;
        }
        n -= 1ll << pos;
        cout << n << '\n';
    } else cout << "impossible";
}

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

//    cin >> t;

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

    return 0;
}