#include<bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
typedef long long ll;
#define For(i,j,k) for (int i=(j);i<=(k);i++)
#define rof(i,j,k) for (int i=(j);i>=(k);i--)
#define ull unsigned long long
#define lowbit(x) ((x)&(-(x)))
#define PII pair<int,int>
#define int long long
#define endl "\n"
#define ls rt<<1
#define rs rt<<1|1
const int mod = 998244353;
const int N=2e6+7;
int n,m;
void solve(){
    cin>>n;
    int x=n-n/3;
    cout<<n*(n-1)/2-x*(x-1)/2<<endl;
}
signed main(){
    int t=1;
    //cin>>t;
    while(t--){
        solve();
    }
}