#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, a, b;
int ans;
int mat[11][11];
int main() {
  scanf("%d%d%d%d", &r, &c, &n, &k);
  memset(mat, 0, sizeof(mat));
  for (int i = 0; i < n; i++) {
    scanf("%d%d", &a, &b);
    mat[a][b] = 1;
  }
  for (int y = 1; y <= r; y++) {
    for (int x = 1; x <= c; x++) {
      mat[y][x] = mat[y][x] + mat[y - 1][x] + mat[y][x - 1] - mat[y - 1][x - 1];
    }
  }
  ans = 0;
  for (int y = 1; y <= r; y++) {
    for (int x = 1; x <= c; x++) {
      for (int y2 = y; y2 <= r; y2++) {
        for (int x2 = x; x2 <= c; x2++) {
          if (mat[y2][x2] - mat[y2][x - 1] - mat[y - 1][x2] +
                  mat[y - 1][x - 1] >=
              k)
            ans++;
        }
      }
    }
  }
  printf("%d\n", ans);
  return 0;
}
