#include <bits/stdc++.h> using namespace std; int main() { long long N,n,an,s=0; cin>>N; if(N%10<5) { n=N/10-1; } else n=N/10; if((N-10)%5>=1) { an=(N-10)/5; } else an=(N-10)/5-1; for(int i=an;i>=0;i-=2) { s=s+i; } cout<<s; return 0; }