#include <bits/stdc++.h>
using namespace std;
int N, arr[100005], a, b, c;
int R, C, K;
int mat[15][15];
int u, v;
int main() {
  int res = 0;
  memset(mat, 0, sizeof(mat));
  scanf("%d %d %d %d", &R, &C, &N, &K);
  for (int i = 0; i < N; i++) {
    scanf("%d %d", &u, &v);
    mat[u][v] = 1;
  }
  for (int i = 1; i <= R; i++) {
    for (int j = 1; j <= C; j++) {
      for (int k = 1; k <= R; k++) {
        for (int l = 1; l <= C; l++) {
          int cnt = 0;
          for (int x = i; x <= k; x++) {
            for (int y = j; y <= l; y++) {
              cnt += mat[x][y];
            }
          }
          if (cnt >= K) {
            res++;
          }
        }
      }
    }
  }
  printf("%d\n", res);
}
