#include <bits/stdc++.h>
using namespace std;
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
const int N = 1e5 + 10;
const int MOD = 1e9 + 7;
const long double eps = 1e-12;
int r, c, n, k;
bool mark[20][29];
bool calc(int r1, int r2, int c1, int c2) {
  int res = 0;
  for (int i = r1; i <= r2; i++)
    for (int j = c1; j <= c2; j++)
      if (mark[i][j]) res++;
  return (res >= k);
}
int main() {
  r = in(), c = in(), n = in(), k = in();
  for (int i = 0; i < n; i++) {
    int a = in(), b = in();
    mark[a][b] = 1;
  }
  int ans = 0;
  for (int i = 1; i <= r; i++)
    for (int j = i; j <= r; j++)
      for (int x = 1; x <= c; x++)
        for (int y = x; y <= c; y++) ans += calc(i, j, x, y);
  cout << ans;
  return 0;
}
