#include <bits/stdc++.h>
using namespace std;
bool a[11][11];
int main() {
  int R, C, N, K;
  scanf("%d%d%d%d", &R, &C, &N, &K);
  for (int i = 0; i < N; i++) {
    int y, x;
    scanf("%d%d", &y, &x);
    y--;
    x--;
    a[y][x] = true;
  }
  int ans = 0;
  for (int i = 0; i < R; i++) {
    for (int j = 0; j < C; j++) {
      for (int r = 1; r <= R; r++) {
        if (i + r > R) break;
        for (int c = 1; c <= C; c++) {
          if (j + c > C) break;
          int cur = 0;
          for (int p = 0; p < r; p++)
            for (int q = 0; q < c; q++) cur += a[i + p][j + q];
          if (cur >= K) ans++;
        }
      }
    }
  }
  printf("%d", ans);
  return 0;
}
