#include<stdio.h>
#include <math.h>

int main()
{


long p, n, k = 2, c= 25;

freopen("fact.in","r",stdin);
freopen("fact.out","w",stdout);

scanf("%ld", &p);

n = 5*p;

while ( n / c >=1)
{
	n= n - 5 * (k-1);
	k++;
	c = c*5;
}

printf("%ld",n);

}