#include <bits/stdc++.h>
using namespace std;
bool O[10][10];
int main() {
  int R, C, N, K;
  cin >> R >> C >> N >> K;
  for (int i = 0; i < N; i++) {
    int x, y;
    cin >> x >> y;
    O[x - 1][y - 1] = true;
  }
  int c = 0;
  for (int i = 0; i < R; i++) {
    for (int j = 0; j < C; j++) {
      for (int w = 0; i + w <= R; w++) {
        for (int h = 0; j + h <= C; h++) {
          int k = 0;
          for (int ii = i; ii < i + w; ii++) {
            for (int jj = j; jj < j + h; jj++) {
              if (O[ii][jj]) k++;
            }
          }
          if (k >= K) c++;
        }
      }
    }
  }
  cout << c << endl;
}
