1023

// OK Timus 1023

# include <iostream>

using namespace std;

int main() {
    size_t n;
    cin >> n;
    size_t res = n - 1;
    for (size_t i = 3; i * i <= n; i++) {
        if (n % i == 0) {
            res = i - 1;
            break;
        }
    }
    if (n % 2 == 0 && n > 4 && n / 2 < res) {
        cout << n / 2 - 1;
    } else {
        cout << res;
    }
    return 0;
}