include <bits/stdc++.h>

using namespace std; int main() {

long long a,n,an,s=0;
cin>>a;
if(a%10<5)
{
	n=a/10-1;
}
else n=a/10;
if((a-10)%5>=1)
{
	an=(a-10)/5;
}
else an=(a-10)/5-1;
for(int i=an;i>=0;i-=2)
{
	s=s+i;
}
	cout<<s;
	return 0;

}