#include <stdio.h>

int main(){
	FILE *fin,*fout;
	fin=fopen("fact.in","rt");
	fout=fopen("fact.out","wt");


	long int N,P,aux;

	fscanf(fin,"%ld",&P);

	if (P<0){
		fprintf(fout,"-1");
		goto close;
	}

	if (P==0){
		fprintf(fout,"1");
		goto close;
	}

	aux=P;
	while(P%5==0){
		aux--;
		P/=5;
	}

	N=aux*5;

	fprintf(fout,"%ld",N);
	close:
	fclose(fin);
	fclose(fout);
	return 0;
}