#include <bits/stdc++.h>
using namespace std;
const int maxn = 30;
int vis[maxn][maxn];
int main() {
  int r, c, n, k;
  int x, y;
  while (scanf("%d%d%d%d", &r, &c, &n, &k) != EOF) {
    memset(vis, 0, sizeof(vis));
    for (int i = 0; i < n; i++) {
      scanf("%d%d", &x, &y);
      vis[x][y] = 1;
    }
    int ans = 0;
    for (int rx = 1; rx <= r; rx++) {
      for (int ry = 1; ry <= c; ry++) {
        for (int i = 1; i + rx - 1 <= r; i++) {
          for (int j = 1; j + ry - 1 <= c; j++) {
            int num = 0;
            for (int x = i; x < i + rx; x++) {
              for (int y = j; y < j + ry; y++) {
                if (vis[x][y]) {
                  num++;
                }
              }
            }
            if (num >= k) ans++;
          }
        }
      }
    }
    printf("%d\n", ans);
  }
  return 0;
}
