#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
inline void amin(T &x, U y) {
  if (y < x) x = y;
}
template <typename T, typename U>
inline void amax(T &x, U y) {
  if (x < y) x = y;
}
int nums[50][50] = {0};
int place[50][50] = {0};
int main() {
  int r, c, n, kk;
  cin >> r >> c >> n >> kk;
  vector<int> x, y;
  for (int i = 0; i < n; i++) {
    int a, b;
    cin >> a >> b;
    place[a][b] = 1;
  }
  for (int i = 1; i <= r; i++) {
    int cnt = 0;
    for (int j = 1; j <= c; j++) {
      if (place[i][j] == 1) cnt++;
      nums[i][j] = nums[i - 1][j] + cnt;
    }
  }
  int ans = 0;
  for (int i = 0; i <= r; i++) {
    for (int j = 0; j <= c; j++) {
      for (int k = i; k <= r; k++) {
        for (int l = j; l <= c; l++) {
          if ((nums[k][l] + nums[i][j] - nums[i][l] - nums[k][j]) >= kk) ans++;
        }
      }
    }
  }
  cout << ans << endl;
}
