inverse factorial

#include <iostream>
#include <cmath>

using namespace std;

string s;

int main() {
    cin >> s;

    if (s == "1") cout << 1 << endl;
    else if (s == "2") cout << 2 << endl;
    else if (s == "6") cout << 3 << endl;
    else if (s == "24") cout << 4 << endl;
    else if (s == "120") cout << 5 << endl;
    else if (s == "720") cout << 6 << endl;
    else {
        double resoult = 0;
        for (int i = 1; i < 1000000; ++i) {
            resoult += log10(i);
            if ( (int)resoult + 1 == s.size() ) {
                cout << i << endl;
                break;
            }
        }
    }


    return 0;
}