#include <bits/stdc++.h>
using namespace std;
const double kEps = 10e-8;
const int kMax = 16;
const int kInf = 1 << 30;
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  int g[kMax][kMax];
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      g[i][j] = 0;
    }
  }
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    g[x][y] = 1;
  }
  int count = 0;
  for (int r1 = 1; r1 <= r; r1++) {
    for (int c1 = 1; c1 <= c; c1++) {
      for (int r2 = r1; r2 <= r; r2++) {
        for (int c2 = c1; c2 <= c; c2++) {
          int sum = 0;
          for (int i = r1; i <= r2; i++) {
            for (int j = c1; j <= c2; j++) {
              sum += g[i][j] ? 1 : 0;
            }
          }
          if (sum >= k) {
            count++;
          }
        }
      }
    }
  }
  cout << count << endl;
  return EXIT_SUCCESS;
}
