#include <fstream>


using namespace std;


ifstream f("fact.in");
ofstream g("fact.out");

long long fact( int n ){
	long long p = 1;
	int i;
	for( i = 2; i <= n; i++ )
		p = p*i;
	return p;
}
int zeros( long long s){
	int p = 0;
	while ( s%10 == 0 ){
		p++;
		s = s/10;
	}
 	return p;

}

long long binarysearch(int n, int p){
	long long lo, hi, mid;
	lo = 0;
	hi = n+1;
	while ( hi - lo > 1){
		mid =( lo + hi ) /2;
			if ( zeros ( fact(mid)) > p )
				hi = mid;
			else
				lo = mid;
	}
	return lo;
}


int main(){
	int p;
	f >> p;	
	if ( p == 0 ){
		g << 1;
		return 0;
	}
    p = binarysearch(5*p, p);
    g << p;

}