#include <bits/stdc++.h>
using namespace std;
int a[100][100];
int main() {
  int n, m, u, v;
  cin >> n >> m >> u >> v;
  for (int x = 0; x < u; ++x) {
    int uu, vv;
    cin >> uu >> vv;
    a[uu][vv] = 1;
  }
  int ans = 0;
  for (int x = 1; x <= n; ++x) {
    for (int y = 1; y <= m; ++y) {
      for (int i = x; i <= n; ++i) {
        for (int j = y; j <= m; ++j) {
          int res = 0;
          for (int ii = x; ii <= i; ++ii)
            for (int jj = y; jj <= j; ++jj) res += a[ii][jj];
          if (res >= v) {
            ans++;
          }
        }
      }
    }
  }
  cout << ans << endl;
}
