#include <iostream>
#include <fstream>
using namespace std;

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

int pow5(int n)
{
    long long s = 0;
    if(n >= 5)
    {
        s+=n/5;
        n/=5;
    }
    return s;
}

int main()
{
    int a = 1;
    long int c,b = 100000000,p;
    f>>p;
    if(p == 0)
        {
            g<<1;
            return 0;
        }
    while(a < b)
    {
        c = (a + b)/2;
        if(pow5(c) > p)
        {
            b = c;
        }
        else
        {
            a = c + 1;
        }
    }
    if(pow5(c) != p)
        g<<-1;
    else
        g<<a-5;
    return 0;
}