#include <bits/stdc++.h>
using namespace std;
int main() {
  int R, C, N, K;
  while (cin >> R >> C >> N >> K) {
    int x[16], y[16];
    for (int i = 0; i < N; ++i) cin >> x[i] >> y[i];
    int count = 0;
    for (int i = 1; i <= R; ++i)
      for (int j = 1; j <= C; ++j)
        for (int k = i; k <= R; ++k)
          for (int l = j; l <= C; ++l) {
            int p = 0;
            for (int n = 0; n < N; ++n)
              if (i <= x[n] && x[n] <= k && j <= y[n] && y[n] <= l) ++p;
            if (p >= K) ++count;
          }
    cout << count << endl;
  }
  return 0;
}
