#include<cstdio>
using namespace std;
int main()
{
	freopen("fact.in","r",stdin);freopen("fact.out","w",stdout);
	int s=0,i,k=5,l=0,a;
	scanf("%d",&a);
	for(i=5;l!=a;i+=5)
	{
		if(i==k)
			k*=5,s++,l+=s;
		else
			l++;
	}
	printf("%d",i-5);
}