#include <bits/stdc++.h>
using namespace std;
long long r, c, n, k, x, y;
int sum, res;
bool mp[20][20];
int main() {
  scanf("%lld %lld %lld %lld", &r, &c, &n, &k);
  for (int i = 1; i <= n; i++) {
    scanf("%lld %lld", &x, &y);
    mp[x][y] = true;
  }
  for (int x1 = 1; x1 <= r; x1++) {
    for (int y1 = 1; y1 <= c; y1++) {
      for (int x2 = x1; x2 <= r; x2++) {
        for (int y2 = y1; y2 <= c; y2++) {
          res = 0;
          for (int i = x1; i <= x2; i++) {
            for (int j = y1; j <= y2; j++) {
              if (mp[i][j]) res++;
            }
          }
          if (res >= k) sum++;
        }
      }
    }
  }
  printf("%d\n", sum);
  return 0;
}
