181B - Number of Triplets - CodeForces Solution

#include<bits/stdc++.h>
#define ll long long
#define fi first
#define sec second
#define pb push_back
#define pqueue priority_queue
#define int long long
using namespace std;
ll t,n,m,ans=0;
string soy;
bool ada[2001][2001];
pair<int,int> a[3001];
main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(0);
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i].fi>>a[i].sec;
		ada[a[i].fi+1000][a[i].sec+1000]=1;
		a[i].fi+=1000;
		a[i].sec+=1000;
	}
	sort(a+1,a+n+1);
	ans=0;
	for(int i=1;i<=n-2;i++){
		for(int j=i+2;j<=n;j++){
			if((a[i].fi+a[j].fi)%2==0&&(a[i].sec+a[j].sec)%2==0){
				if(ada[(a[i].fi+a[j].fi)/2][(a[i].sec+a[j].sec)/2]){
					ans++;
				}
			}
		}
	}
	cout<<ans<<endl;
}