#include <bits/stdc++.h>
using namespace std;
const int maxn = 10 + 5, oo = 0x3f3f3f3f;
int a[maxn], b[maxn];
int n;
int main(void) {
  int r, c, n, kk;
  cin >> r >> c >> n >> kk;
  for (int i = 0; i < n; i++) {
    cin >> a[i] >> b[i];
  }
  int cnt = 0;
  int ans = 0;
  for (int i = 1; i <= r; i++) {
    for (int ii = i; ii <= r; ii++) {
      for (int j = 1; j <= c; j++) {
        for (int jj = j; jj <= c; jj++) {
          cnt = 0;
          for (int k = 0; k < n; k++) {
            if (a[k] >= i && a[k] <= ii && b[k] >= j && b[k] <= jj) cnt++;
          }
          if (cnt >= kk) ans++;
        }
      }
    }
  }
  printf("%d\n", ans);
  return 0;
}
