#include <iostream>
#include <fstream>

using namespace std;

ifstream fin("fact.in");
ofstream fout("fact.out");

int zero(int x)
{
    int i,k=0; long long p=1;
    for(i=1;i<=x;i++)
        p*=i;
    while(p%10 == 0){k++; p=p/10;}
    return k;
}

int cb(int p)
{
    int st=1,dr=5*p;
    while(st<=dr)
    {
        int m,r;
        m=(st+dr)/2;
        r=zero(m);
        if(r == p)return m;
        else
            if(r < p)st=m+1;
            else
                dr=m-1;
    }
    return -1;
}

int main()
{
    int x; fin>>x;
    if(x==0)fout<<1;
    else
        fout<<cb(x);
    return 0;
}