#include <bits/stdc++.h>
using namespace std;
int main() {
  int i1, j1, i, x = 0, j, k, l, n, m, c, r, a[20][20];
  cin >> c >> r >> n >> k;
  for (i = 1; i <= 10; i++)
    for (j = 1; j <= 10; j++) a[i][j] = 0;
  for (j = 1; j <= n; j++) {
    cin >> m >> l;
    a[m][l] = 1;
  }
  for (i = 1; i <= c; i++)
    for (j = 1; j <= r; j++) {
      for (i1 = i; i1 <= c; i1++)
        for (j1 = j; j1 <= r; j1++) {
          int q = 0;
          for (int i2 = i; i2 <= i1; i2++)
            for (int j2 = j; j2 <= j1; j2++) q = q + a[i2][j2];
          if (q >= k) x++;
        }
    }
  cout << x;
}
