#include <bits/stdc++.h>
using namespace std;
const int N = 15;
const int K = 500;
int a[N][N];
const int INF = 1e9 + 7;
int calc(int x, int y, int h, int w) {
  int cnt = 0;
  for (int i = x; i < x + h; i++) {
    for (int j = y; j < y + w; j++) {
      cnt += a[i][j];
    }
  }
  return cnt;
}
int main() {
  int r, c;
  int n, k;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    a[x][y] = 1;
  }
  int ans = 0;
  for (int d = 1; d <= r; d++) {
    for (int w = 1; w <= c; w++) {
      for (int i = 0; i + d - 1 < r; i++) {
        for (int j = 0; j + w - 1 < c; j++) {
          if (calc(i, j, d, w) >= k) ans++;
        }
      }
    }
  }
  cout << ans;
}
