#include <bits/stdc++.h>
using namespace std;
const int N = 505;
int r, c, n, k, x, y, grid[N][N], cnt, ans;
int main() {
  scanf("%d %d %d %d", &r, &c, &n, &k);
  for (int i = 0; i < (int)(n); i++) {
    scanf("%d %d", &x, &y);
    grid[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int p = i; p <= r; p++) {
        for (int q = j; q <= c; q++) {
          cnt = 0;
          for (int a = i; a <= p; a++) {
            for (int b = j; b <= q; b++) {
              cnt += grid[a][b];
            }
          }
          if (cnt >= k) ans++;
        }
      }
    }
  }
  printf("%d\n", ans);
  return 0;
}
