#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, ans;
bool mp[22][22];
int sum[22][22];
int main() {
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (int i = 1; i <= n; i++) {
    int x, y;
    scanf("%d%d", &x, &y);
    mp[x][y] = 1;
  }
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++)
      sum[i][j] = sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1] + mp[i][j];
  for (int a1 = 1; a1 <= r; a1++)
    for (int b1 = 1; b1 <= c; b1++)
      for (int a2 = a1; a2 <= r; a2++)
        for (int b2 = b1; b2 <= c; b2++) {
          int t = sum[a2][b2] - sum[a2][b1 - 1] - sum[a1 - 1][b2] +
                  sum[a1 - 1][b1 - 1];
          if (t >= k) ans++;
        }
  printf("%d\n", ans);
  return 0;
}
