Solution

#include <iostream>
#include <algorithm>
using namespace std;
 
typedef pair<int,int> Point;
bool ycomp(Point p, Point q) { return p.second < q.second; }
 
const int MAX_N = 2500;
int N, Psum[MAX_N+1][MAX_N+1];
Point P[MAX_N];
 
int rsum(int x1, int y1, int x2, int y2)
{
  return Psum[x2+1][y2+1] - Psum[x2+1][y1] - Psum[x1][y2+1] + Psum[x1][y1];
}
 
int main(void)
{
  cin >> N;
  for (int i=0; i<N; i++) {
    int x, y;
    cin >> x >> y;
    P[i] = make_pair(x,y);
  }
  sort(P, P+N); 
  for (int i=0; i<N; i++) P[i].first = i+1;
  sort(P, P+N, ycomp); 
  for (int i=0; i<N; i++) P[i].second = i+1;
  for (int i=0; i<N; i++) Psum[P[i].first][P[i].second] = 1;
  for (int i=1; i<=N; i++)
    for (int j=1; j<=N; j++)
      Psum[i][j] += Psum[i-1][j] + Psum[i][j-1] - Psum[i-1][j-1];
  long long answer = 0;
  for (int i=0; i<N; i++) 
    for (int j=i; j<N; j++) {
      int x1 = min(P[i].first, P[j].first) - 1;
      int x2 = max(P[i].first, P[j].first) - 1;
      answer += rsum(0,i,x1,j) * rsum(x2,i,N-1,j);
    }
  cout << answer + 1 << "\n";
  
}