#include <bits/stdc++.h>
using namespace std;
const int INF = 1000 * 1000 * 1000;
const long long INF64 = 1LL * INF * INF;
const long long mod = INF + 7;
int m[10][10];
bool check(int k, int x, int y, int w, int h) {
  int ans = 0;
  for (int i = x; i <= h; ++i) {
    for (int j = y; j <= w; ++j) {
      ans += m[i][j];
    }
  }
  return ans >= k;
}
void solve() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; ++i) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    m[x][y] = 1;
  }
  int ans = 0;
  for (int i = 0; i < r; ++i) {
    for (int j = 0; j < c; ++j) {
      for (int w = j; w < c; ++w) {
        for (int h = i; h < r; ++h) {
          ans += check(k, i, j, w, h);
        }
      }
    }
  }
  cout << ans << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  solve();
  return 0;
}
