#include <bits/stdc++.h>
using namespace std;
int n, g[20][20], r, c, k, ans = 0, x, y;
bool chk(int a, int b, int c, int d) {
  int cnt = 0;
  for (int i = a; i <= a + c; i++) {
    for (int j = b; j <= b + d; j++) {
      cnt += g[i][j];
      if (cnt >= k) return 1;
    }
  }
  return 0;
}
int main() {
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (int i = 0; i < n; i++) {
    scanf("%d%d", &x, &y);
    g[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int e = 0; e + i <= r; e++) {
        for (int f = 0; f + j <= c; f++) {
          ans += chk(i, j, e, f);
        }
      }
    }
  }
  cout << ans;
  return 0;
}
