#include <iostream>

using namespace std;
int main(){
    int age = 0;
    int result = -1;
    cin >> age;
    for(int x = 2;x < age;x++){
        int c_result = (age % x) + (age / x) * 10;
        if(c_result == 17){ result = x;x = age;}
    }
    cout << result << endl;
}