Sum

#include <iostream>
#include <vector>
#include <stack>
#include <unordered_set>
#include <numeric>
#include <iterator>
#include <unordered_map>
#include <array>
#include <map>
#include <cstring>
#include <iomanip>
#include <string>
#include <algorithm>
#include <cmath>
#include <ctgmath>
#include <set>
#include <queue>
#include <fstream>
#include <ostream>
#include <list>
#include <string>
using namespace std;

signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	int n;
	cin >> n;

	int cnt = 0;
	for (int i = 1; i < n; i++)
	{
		for (int j = i + 1; j < n; j++)
		{
			for (int k = j + 1; k < n; k++)
			{
				if (i + j + k == n)
				{
					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