#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    ifstream f("fact.in");
    ofstream g("fact.out");

    int p;
    f>>p;

    int fivething = 1;
    int exponent = 0;

    for(int i = 0; i < 16; i++){
      if(fivething*5 <= p) {
        fivething *= 5;
        exponent++;
      }
      else break;
    }

    if(p>=5)
      g<<p * 5 -  5 * ((exponent + 1) * exponent)/2;


    else
      g<<1;

    return 0;
}