#include <bits/stdc++.h>
using namespace std;
int n, m, ans, r, c, x, y;
bool used[11][11];
bool istrue(int a, int b, int c, int d) {
  int i, j, cnt = 0;
  for (i = a; i <= c; i++) {
    for (j = b; j <= d; j++) {
      cnt += used[i][j];
    }
  }
  return cnt >= m;
}
int main() {
  int i, j, h, k;
  cin >> r >> c >> n >> m;
  for (i = 1; i <= n; i++) {
    cin >> x >> y;
    used[x][y] = true;
  }
  for (i = 1; i <= r; i++) {
    for (j = 1; j <= c; j++) {
      for (h = i; h <= r; h++) {
        for (k = j; k <= c; k++) {
          if (istrue(i, j, h, k)) ans++;
        }
      }
    }
  }
  cout << ans << endl;
}
