#include <bits/stdc++.h>
using namespace std;
int a[11][11];
int main() {
  int r, c, n, k, ant;
  while (cin >> r >> c >> n >> k) {
    ant = 0;
    for (int i = 0; i < n; i++) {
      int x, y;
      cin >> x >> y;
      a[x - 1][y - 1] = 1;
    }
    for (int i = 0; i < r; i++)
      for (int j = 0; j < c; j++)
        for (int i1 = 0; i1 < r; i1++)
          for (int j1 = 0; j1 < c; j1++) {
            int sum = 0;
            for (int i2 = i; i2 <= i1; i2++)
              for (int j2 = j; j2 <= j1; j2++) {
                sum += a[i2][j2];
              }
            if (sum >= k) ant++;
          }
    cout << ant << endl;
  }
  return 0;
}
