#include <bits/stdc++.h>
using namespace std;
const int MAX = 10 + 1;
int x[MAX], y[MAX];
int main() {
  int r, c, n, k;
  while (scanf("%d%d%d%d", &r, &c, &n, &k) != EOF) {
    for (int i = 0; i < n; i++) {
      scanf("%d%d", &x[i], &y[i]);
    }
    int ans = 0;
    for (int rs = 1; rs <= r; rs++) {
      for (int re = rs; re <= r; re++) {
        for (int cs = 1; cs <= c; cs++) {
          for (int ce = cs; ce <= c; ce++) {
            int num = 0;
            for (int i = 0; i < n; i++) {
              if (x[i] >= rs && x[i] <= re && y[i] >= cs && y[i] <= ce) num++;
            }
            if (num >= k) ans++;
          }
        }
      }
    }
    printf("%d\n", ans);
  }
  return 0;
}
