#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k;
  scanf("%d%d%d%d\n", &r, &c, &n, &k);
  int a[r][c];
  for (int i = 0; i < r; i++) {
    for (int j = 0; j < c; j++) {
      a[i][j] = 0;
    }
  }
  int x, y;
  for (int i = 0; i < n; i++) {
    scanf("%d %d\n", &x, &y);
    a[x - 1][y - 1] = 1;
  }
  int ans = 0, lsum;
  for (int r1 = 0; r1 < r; r1++)
    for (int r2 = r1 + 1; r2 < r + 1; r2++)
      for (int c1 = 0; c1 < c; c1++)
        for (int c2 = c1 + 1; c2 < c + 1; c2++) {
          lsum = 0;
          for (int i = r1; i < r2; i++)
            for (int j = c1; j < c2; j++) lsum += a[i][j];
          if (lsum >= k) ans++;
        }
  printf("%d\n", ans);
  return 0;
}
