#include <bits/stdc++.h>
using namespace std;
int n, c, r, k, a[20][20], ans, b[20][20], d[20][20];
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    a[x][y] = b[x][y] = d[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      b[i][j] += b[i - 1][j];
      d[i][j] += d[i][j - 1];
    }
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      a[i][j] += a[i - 1][j - 1];
      a[i][j] += b[i - 1][j];
      a[i][j] += d[i][j - 1];
    }
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int q = i; q <= r; q++) {
        for (int u = j; u <= c; u++) {
          if (a[q][u] - a[i - 1][u] - a[q][j - 1] + a[i - 1][j - 1] >= k) ans++;
        }
      }
    }
  }
  cout << ans;
  return 0;
}
