#include <bits/stdc++.h>
using namespace std;
int r, c, n, k;
int mat[13][13];
int sum[13][13];
int cnt;
int main() {
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (int i = 0, x, y; i < n; i++) {
    scanf("%d%d", &x, &y);
    mat[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      sum[i][j] = mat[i][j];
      sum[i][j] += sum[i - 1][j];
      sum[i][j] += sum[i][j - 1];
      sum[i][j] -= sum[i - 1][j - 1];
    }
  }
  for (int i = 0; i < r; i++) {
    for (int i2 = i + 1; i2 <= r; i2++) {
      for (int j = 0; j < c; j++) {
        for (int j2 = j + 1; j2 <= c; j2++) {
          int s = sum[i2][j2] - sum[i2][j] - sum[i][j2] + sum[i][j];
          if (s >= k) cnt++;
        }
      }
    }
  }
  printf("%d", cnt);
  return 0;
}
