#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, x, y, ans, t;
int a[15][15];
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int f = i; f <= r; f++) {
        for (int e = j; e <= c; e++) {
          t = 0;
          for (int x = i; x <= f; x++) {
            for (int y = j; y <= e; y++) t += a[x][y];
          }
          if (t >= k) ans++;
        }
      }
    }
  }
  cout << ans;
  return 0;
}
