#include <fstream>
using namespace std;
int P, nrz = 1, N = 1;

int zero(int k){
    long m = 5;
    int z = 0;
    while(k/m != 0){
        z+=k/m;
        m*=5;
    }
    return z;
}

int searchN(){
    while(nrz != P){
    nrz = zero(N);
    N++;
    }
    return N;
}

int main()
{
    ifstream f("fact.in");
    ofstream g("fact.out");
    f>>P;
    g<<searchN() - 1;
    return 0;
}