#include <stdio.h>

void div25(long x,int* k1,int* k2,long* P)
{
	while(x%2==0)
	 {
	 	if(*(k1)!=*P) (*k1)++;
	 	x/=2;
	 }
	 while(x%5==0)
	 {
	 	if(*(k2)!=*P) (*k2)++;
	 	x/=5;
	 }
}

int main()
{
	long i=2,P;
	int k1=0,k2=0;
	
	FILE *fp;
	fp=fopen("fact.in","r");
	fscanf(fp,"%ld",&P);
	fclose(fp);
	fp=fopen("fact.out","w");
	
	while(k1+k2!=2*P)
	 {
	 	div25(i,&k1,&k2,&P);
	 	i++;
	 }
	 fprintf(fp,"%ld",i-1);
}