#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, x, y;
int flag[101][101];
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    flag[x][y] = 1;
  }
  int cnt = 0;
  for (int i = 1; i <= r; i++) {
    for (int i1 = 1; i1 <= c; i1++) {
      for (int i2 = 1; i2 <= r; i2++) {
        for (int i3 = 1; i3 <= c; i3++) {
          int sum = 0;
          for (int i5 = i; i5 <= i2; i5++) {
            for (int i6 = i1; i6 <= i3; i6++) {
              if (flag[i5][i6] == 1) sum++;
            }
          }
          if (sum >= k) cnt++;
        }
      }
    }
  }
  cout << cnt;
  return 0;
}
