#include <bits/stdc++.h>
using namespace std;
int p[20][20];
int cnt(int x1, int y1, int x2, int y2) {
  int res = 0;
  for (int i = x1; i <= x2; i++) {
    for (int j = y1; j <= y2; j++) {
      res += p[i][j];
    }
  }
  return res;
}
int main() {
  int r, c, n, k;
  scanf("%d %d", &r, &c);
  scanf("%d %d", &n, &k);
  for (int i = 0; i < n; i++) {
    int x, y;
    scanf("%d %d", &x, &y);
    p[x][y] = 1;
  }
  int res = 0;
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int x = i; x <= r; x++) {
        for (int y = j; y <= c; y++) {
          if (cnt(i, j, x, y) >= k) res++;
        }
      }
    }
  }
  printf("%d", res);
  return 0;
}
