#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, a, b, cnt, d[25][25], s[25][25];
long long ans;
int main() {
  int i, j, g, w, u;
  cin >> r >> c >> n >> k;
  for (i = 0; i < n; i++) {
    scanf("%d %d", &a, &b);
    d[a][b] = 1;
  }
  for (i = 1; i <= r; i++) {
    for (j = 1; j <= c; j++) {
      s[i][j] = s[i][j - 1] + d[i][j];
    }
  }
  for (i = 1; i <= r; i++) {
    for (j = i; j <= r; j++) {
      for (g = 1; g <= c; g++) {
        for (w = 0; w < g; w++) {
          cnt = 0;
          for (u = i; u <= j; u++) {
            cnt += s[u][g] - s[u][w];
          }
          if (cnt >= k) ans++;
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
