#include <bits/stdc++.h>
int a[10][10] = {0};
int main(void) {
  int r, c, n, k, i, j, p, q, s = 0, ans = 0, x, y, f, g;
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (i = 0; i < n; i++) {
    scanf("%d%d", &x, &y);
    a[x - 1][y - 1] = 1;
  }
  for (i = 0; i < r; i++)
    for (j = 0; j < c; j++) {
      for (p = j; p < c; p++) {
        s = 0;
        for (q = j; q <= p; q++)
          if (a[i][q] == 1) s++;
        if (s >= k) ans++;
      }
      for (p = i + 1; p < r; p++) {
        s = 0;
        for (q = i; q <= p; q++)
          if (a[q][j] == 1) s++;
        if (s >= k) ans++;
      }
      for (p = j + 1; p < c; p++)
        for (q = i + 1; q < r; q++) {
          s = 0;
          for (f = j; f <= p; f++)
            for (g = i; g <= q; g++)
              if (a[g][f] == 1) s++;
          if (s >= k) ans++;
        }
    }
  printf("%d", ans);
  return 0;
}
