#include <bits/stdc++.h>
using namespace std;
bool bi[12][12];
int main() {
  int a, b, c, d, e, x, y, z, n, k, N, M;
  scanf("%d %d", &N, &M);
  scanf("%d %d", &z, &k);
  for (a = 0; a < z; a++) {
    scanf("%d %d", &x, &y);
    bi[x][y] = true;
  }
  z = 0;
  for (a = 1; a <= N; a++) {
    for (b = 1; b <= M; b++) {
      for (c = a; c <= N; c++) {
        for (d = b; d <= M; d++) {
          e = 0;
          for (x = a; x <= c; x++) {
            for (y = b; y <= d; y++)
              if (bi[x][y]) e++;
          }
          if (e >= k) z++;
        }
      }
    }
  }
  cout << z << endl;
  return 0;
}
