1938.cpp

// Ivan Carvalho
// Solution to https://www.beecrowd.com.br/judge/problems/view/1938
#include <algorithm>
#include <cstdio>
#include <vector>
#define X second
#define Y first
#define MP make_pair
using namespace std;
typedef pair<int, int> point;
const int NEG = -(1e8 + 1);
int n, resp;
vector<point> pontos;
bool comp(point A, point B) {
    if (A.X == B.X) return A.Y < B.Y;
    return A.X < B.X;
}
int main() {
    while (scanf("%d", &n) != EOF) {
        resp = 0;
        pontos.clear();
        for (int i = 1; i <= n; i++) {
            int x, y;
            scanf("%d %d", &x, &y);
            pontos.push_back(MP(y, x));
        }
        sort(pontos.begin(), pontos.end(), comp);
        for (int i = 0; i < n; i++) {
            point agora = pontos[i];
            int MIN = NEG;
            int MAX = -MIN;
            for (int j = i + 1; j < n; j++) {
                point proximo = pontos[j];
                if (proximo.Y > MIN && proximo.Y < MAX &&
                    proximo.Y != agora.Y) {
                    resp++;
                }
                if (proximo.Y > agora.Y) {
                    MAX = min(MAX, proximo.Y);
                }
                if (proximo.Y < agora.Y) {
                    MIN = max(proximo.Y, MIN);
                }
            }
        }
        printf("%d\n", resp);
    }
    return 0;
}