#include <bits/stdc++.h>
using namespace std;
string ITS(long long x) {
  string s = "";
  while (x > 0) {
    s += (char)(x % 10 + '0');
    x /= 10;
  }
  string t = "";
  for (int i = s.size() - 1; i > -1; i--) t += s[i];
  return t;
}
long long inf = 1e9;
long long mod = 1e9 + 7;
const long long M = 2e1;
bool jad[M][M];
int main() {
  cin.sync_with_stdio(false);
  long long m, n, t, k;
  cin >> n >> m >> t >> k;
  for (long long i = 0; i < t; i++) {
    long long x, y;
    cin >> x >> y;
    x--;
    y--;
    jad[x][y] = 1;
  }
  long long cnt = 0;
  for (long long x = 0; x < n; x++) {
    for (long long y = 0; y < m; y++) {
      for (long long i = 0; i < n - x; i++) {
        for (long long j = 0; j < m - y; j++) {
          long long tc = 0;
          for (long long p = 0; p <= x; p++) {
            for (long long q = 0; q <= y; q++) {
              tc += jad[i + p][j + q];
            }
          }
          if (tc >= k) cnt++;
        }
      }
    }
  }
  cout << cnt;
}
