#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, x, y, ans;
int s[20][20];
int main() {
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (int i = 1; i <= r; ++i)
    for (int j = 1; j <= c; ++j) s[i][j] = 0;
  for (int i = 0; i < n; ++i) {
    scanf("%d%d", &x, &y);
    s[x][y] = 1;
  }
  for (int i = 1; i <= r; ++i)
    for (int j = 1; j <= c; ++j)
      s[i][j] += s[i - 1][j] + s[i][j - 1] - s[i - 1][j - 1];
  for (int i = 1; i <= r; ++i) {
    for (int j = 1; j <= c; ++j)
      for (int h = i; h <= r; ++h)
        for (int x = j; x <= c; ++x) {
          int cnt = s[h][x] - s[i - 1][x] - s[h][j - 1] + s[i - 1][j - 1];
          ans += (cnt >= k);
        }
  }
  printf("%d", ans);
  return 0;
}
