#include <bits/stdc++.h>
using namespace std;
int r, c, n, k;
struct node {
  int x, y;
} a[1001];
int matrix[1001][1001], s[1001][1001], tot = 0;
int sum(int x, int y) {
  int cnt = 0;
  for (int i = 1; i <= x; i++)
    for (int j = 1; j <= y; j++) {
      cnt += matrix[i][j];
    }
  return cnt;
}
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a[i].x >> a[i].y;
    matrix[a[i].x][a[i].y] = 1;
  }
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++) {
      s[i][j] = s[i][j - 1] + s[i - 1][j] - s[i - 1][j - 1];
      if (matrix[i][j] == 1) s[i][j]++;
    }
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++)
      for (int a = i; a <= r; a++)
        for (int b = j; b <= c; b++) {
          if (s[a][b] - s[i - 1][b] - s[a][j - 1] + s[i - 1][j - 1] >= k) tot++;
        }
  printf("%d", tot);
  return 0;
}
