Sum

#include <bits/stdc++.h>
#define va first
#define vb second
#define lb lower_bound
#define pb push_back
#define all(v) v.begin(),v.end()
#define sz(v) ((int)(v).size())
#define fio ios_base::sync_with_stdio(0);cin.tie(0);
using namespace std;
using ll = long long;
using lf = long double;
using pii = pair<int,int>;
using pll = pair<ll,ll>;

int n;
int main(){
    cin >> n;
    int cnt = 0;
    for(int i = 1; i <= n; i++){
        for(int j = i + 1; i + j <= n; j++){
            if(n-i-j > j) ++cnt;
        }
    }
    cout << cnt << '\n';
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: ACCEPTED

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED