#include <iostream>

using namespace std;

int main(){
  int A, ans;

  cin >> A;

  for(int i = 8;i < 100;i++){
    if(A ==  i * 1  + 7){
      ans = i;
      break;
    } else if(i == 99){
      ans = -1;
    }
  }

  cout << ans << endl;
}