#include <fstream>
#include <iostream>

using namespace std;

unsigned long long int p;
unsigned long long int number = 1;
unsigned long long int n;

unsigned long long int fact(unsigned long long int num)
{
    n = 0;
    while(num != 0)
        {num/=5; n += num;}

    return n;
}


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

    f>>p;

    if(p != 0)
    {
        number = p*5;

        while(fact(number-1) >= p)
            number--;
    }
    g<<number;

    return 0;
}