#include <fstream>

using namespace std;

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

int p;

long long zero (long long val)
{
    long long nr0 = 0, pp;
    pp = 5;
    while (val >= pp)
    {
        nr0 = nr0 + (val / pp);
        pp = pp * 5;
    }
    return nr0;
}
long long cautbin (long long nr0)
{
    long long step = 1, i;
    while (step < 5 * p) step <<= 1;

    for (i = 0; step; step >>= 1)
    {
        if (i + step <= 5 * p && zero(i + step) < p)
        {
            i += step;
        }
    }

    return i+1;
}
int main()
{
    f >> p;

    g << cautbin(p) << '\n';
    return 0;
}