#include <bits/stdc++.h>
using namespace std;
inline long long tavan(long long a, long long n, long long mod) {
  long long p = 1;
  while (n > 0) {
    if (n % 2) {
      p = p * a;
      p %= mod;
    }
    n >>= 1;
    a *= a;
    a %= mod;
  }
  return p % mod;
}
long long r, c, n, m, a[(20)][(20)], ans;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> r >> c >> n >> m;
  for (int i = 0; i < n; i++) {
    long long x, y;
    cin >> x >> y;
    a[x][y]++;
  }
  for (int i = 1; i <= r; i++)
    for (int i2 = i; i2 <= r; i2++)
      for (int j = 1; j <= c; j++)
        for (int j2 = j; j2 <= c; j2++) {
          long long cnt = 0;
          for (int k = i; k <= i2; k++)
            for (int q = j; q <= j2; q++) cnt += a[k][q];
          if (cnt >= m) ans++;
        }
  cout << ans;
  return 0;
}
