#include <bits/stdc++.h>
using namespace std;
int main() {
  int R, C, N, K;
  cin >> R >> C >> N >> K;
  bool A[11][11] = {};
  for (int i = 0; i < N; i++) {
    int X, Y;
    cin >> X >> Y;
    A[X][Y] = true;
  }
  int result = 0;
  for (int rowSize = 0; rowSize < R; rowSize++) {
    for (int columnSize = 0; columnSize < C; columnSize++) {
      for (int rowStart = 1; rowStart + rowSize <= R; rowStart++) {
        for (int columnStart = 1; columnStart + columnSize <= C;
             columnStart++) {
          int sum = 0;
          for (int i = rowStart; i <= rowStart + rowSize; i++) {
            for (int j = columnStart; j <= columnSize + columnStart; j++) {
              sum += A[i][j];
            }
          }
          if (sum >= K) result++;
        }
      }
    }
  }
  cout << result;
  return 0;
}
