#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, cnt, ans, a[101], b[101];
int main() {
  while (scanf("%d%d%d%d", &r, &c, &n, &k) != EOF) {
    ans = 0, memset(a, 0, sizeof(a));
    for (int i = 1; i <= n; i++) {
      scanf("%d%d", &a[i], &b[i]);
    }
    for (int i = 1; i <= r; i++) {
      for (int j = 1; j <= c; j++) {
        for (int g = i; g <= r; g++) {
          for (int h = j; h <= c; h++) {
            cnt = 0;
            for (int l = 1; l <= n; l++)
              if (a[l] >= i && a[l] <= g && b[l] >= j && b[l] <= h) cnt++;
            if (cnt >= k) ans++;
          }
        }
      }
    }
    printf("%d\n", ans);
  }
  return 0;
}
