#include <stdio.h>

int main() {

	int P, N = 1, nr_cifre = 0, nr = 1, k = 1;

	FILE *in = fopen("fact.in","rt");
	FILE *out = fopen("fact.out", "wt");
	while(fscanf(in, "%d", &P) != EOF) {
		while(nr_cifre < P) {
			nr *= (k + 1);
			if (nr % 10 == 0) {
				nr_cifre++;
				nr /= 10;
			}
			k++;
		}
		fprintf(out, "%d\n",  k);	

	}
	fclose(in);
	fclose(out);
	return 0;
}