#include <bits/stdc++.h>
using namespace std;
int a[100][100];
int main() {
  int n, r, c, kk, ans = 0;
  cin >> r >> c >> n >> kk;
  for (int i = 0, x, y; i < n; i++) {
    cin >> x >> y;
    a[x][y]++;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int k = i; k <= r; k++) {
        for (int l = j; l <= c; l++) {
          int t = 0;
          for (int f1 = i; f1 <= k; f1++)
            for (int f2 = j; f2 <= l; f2++) t += a[f1][f2];
          if (t >= kk) ans++;
        }
      }
    }
  }
  cout << ans;
}
