#include <bits/stdc++.h>
using namespace std;
const int MaxN = 10 + 10;
char c[MaxN][MaxN];
int main() {
  int n, m, t, k;
  cin >> n >> m >> t >> k;
  for (int i = 0; i < t; ++i) {
    int x, y;
    cin >> x >> y;
    --x;
    --y;
    c[x][y] = '#';
  }
  int ans = 0;
  for (int r1 = 0; r1 < n; ++r1)
    for (int r2 = r1; r2 < n; ++r2)
      for (int c1 = 0; c1 < m; ++c1)
        for (int c2 = c1; c2 < m; ++c2) {
          int cnt = 0;
          for (int i = r1; i <= r2; ++i)
            for (int j = c1; j <= c2; ++j) cnt += (c[i][j] == '#');
          ans += (cnt >= k);
        }
  cout << ans;
}
