#include <bits/stdc++.h>
using namespace std;
int n, m, T, K, ans, now, x, y;
bool f[15][15];
int main() {
  cin >> n >> m >> T >> K;
  memset(f, false, sizeof(f));
  for (int i = 1; i <= T; i++) cin >> x >> y, f[x][y] = true;
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= m; j++)
      for (int i1 = i; i1 <= n; i1++)
        for (int j1 = j; j1 <= m; j1++) {
          now = 0;
          for (int i2 = i; i2 <= i1; i2++)
            for (int j2 = j; j2 <= j1; j2++)
              if (f[i2][j2]) now++;
          if (now >= K) ans++;
        }
  cout << ans << endl;
  return 0;
}
