#include<fstream.h>
#include<iostream.h>
int factorial(int n){int i,f=1;
    for(i=1;i<=n;i++)
	f=f*i;
    return f;}
int seek(int p)
{int m,n=1,i=0;
m=factorial(n);
while(m%10==0)
{
              i++;
               m=m/10;
               }
               if(i==p){return n;}
               else
			   n++;
}
int main(){int p,j;
    fstream f("fact.in",ios::in);
    while(!f.eof())
	f>>p;
    f.close();
    j=seek(p);
    fstream g("fact.out",ios::out);
    g<<j;
    g.close();}