#include <stdio.h>
#include <stdlib.h>
#include <limits.h>




int main()
{

    int p, n, k, m;
     unsigned long long  f;

    FILE *in = fopen("fact.in", "r");
    FILE *out = fopen("fact.out", "w");

    f=1;
    k=0;
    fscanf(in,"%d", p);
     for(n=1; n<=20; n++)
        {



        f*=n;
        if(f%10==0)
            {
                k++;
                if(k==p)
                {

                    break;

                }

            }

        }
        fprintf(out, "%d", n);


return 0;




}