#include <bits/stdc++.h>
using namespace std;
int n, r, c, k, sum = 0;
bool flag[15][15];
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 1; i <= n; i++) {
    int x, y;
    cin >> x >> y;
    flag[x][y] = 1;
  }
  for (int i1 = 1; i1 <= r; i1++) {
    for (int j1 = 1; j1 <= c; j1++)
      for (int i2 = i1; i2 <= r; i2++)
        for (int j2 = j1; j2 <= c; j2++) {
          int ans = 0;
          for (int x = i1; x <= i2; x++)
            for (int y = j1; y <= j2; y++)
              if (flag[x][y]) ans++;
          if (ans >= k) sum++;
        }
  }
  cout << sum << endl;
  return 0;
}
