#include <stdio.h>
int main(){
	FILE*in  = fopen("fact.in","r");
	FILE*out = fopen("fact.out","w");
	long long i,nz,fact,p;

	fscanf(in, "%lld", &p);

	i    = 1;
	nz   = 0;
	fact = 1;

	while (nz<p){
		fact*=i;
		while (fact%10 == 0){
		      fact/=10;
		      nz++;
		}
		fact%=10;
		i++;
	}
	fprintf (out, "%lld", i-1);
	return 0;
}