#include<iostream>
#include<fstream>
using namespace std;

long citire(){
	long p;
	ifstream in("fact.in");
	in>>p;
	in.close();
	return p;
}

int main(){
	long mom=1,i=0,p=citire();
	while(p>0){
		i++;
		mom=mom*i;
		if(mom%10==0){
			p--;
			mom=mom/10;
		}
	}
	ofstream out("fact.out");
	if(i==0)out<<1;
	else out<<i;
	out.close();
	return 0;
}