#include <fstream>
using namespace std;
int main ()
{
    long long int i, j = 1, n = 1, F = 1;
    
    ifstream fin ("fact.in");
    fin >> n;
    fin.close();
    for ( i = 1; i <= n; i++ )
    {
        j *= 10;
    }
    if ( n < 0 || n > 1000000000 )
    {
         ofstream fout("fact.out");
         fout << "Numarul trebuie sa fie cuprins in intervalul [0,1.000.000.000]";
         fout.close();
    }
    else
    {
        for ( i = 1; F % j != 0; i++ )
        {
            F = i * F;
        }
        i--;
        ofstream fout("fact.out");
        fout << i;
        fout.close();
    }
    return 0;
}