#include <bits/stdc++.h>
using namespace std;
const int N = 1e5;
const int inf = 1e9 + 7;
const long long INF = 1e18 + 7;
int r, c, n, l, cp[15][15], x[15], y[15];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> r >> c >> n >> l;
  for (int i = 0; i < n; i++) {
    cin >> x[i] >> y[i];
    cp[x[i]][y[i]] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 2; j <= c; j++) {
      cp[i][j] += cp[i][j - 1];
    }
  }
  for (int i = 1; i <= c; i++) {
    for (int j = 2; j <= r; j++) {
      cp[j][i] += cp[j - 1][i];
    }
  }
  int ans = 0;
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int k = i; k <= r; k++) {
        for (int q = j; q <= c; q++) {
          int p = cp[k][q] - cp[k][j - 1] - cp[i - 1][q] + cp[i - 1][j - 1];
          if (p >= l) ans++;
        }
      }
    }
  }
  cout << ans;
}
