#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;
using ll = long long;
const int INF = (int)1e9 + 7, mod = (int)1e9 + 9, pw = 31, N = (int)1e5 + 123,
          M = (int)1e6 + 123;
const double eps = 1e-11;
const long long inf = 1e18;
int n, m, a[20][20], d[20][20], c, k;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cin >> n >> m >> c >> k;
  for (int i = 1; i <= c; i++) {
    int x, y;
    cin >> x >> y;
    a[x][y]++;
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      d[i][j] = d[i - 1][j] + d[i][j - 1] + a[i][j] - d[i - 1][j - 1];
    }
  }
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      for (int i1 = i; i1 <= n; i1++) {
        for (int j1 = j; j1 <= m; j1++) {
          if (d[i1][j1] - d[i1][j - 1] - d[i - 1][j1] + d[i - 1][j - 1] >= k)
            ans++;
        }
      }
    }
  }
  cout << ans;
  return 0;
}
