#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 10 + 5, MAX_C = 10 + 5, MAX_R = 10 + 5;
bool a[MAX_R][MAX_C];
int x[MAX_N], y[MAX_N];
int main() {
  int r, c;
  cin >> r >> c;
  int n, k;
  cin >> n >> k;
  for (int i = 0; i < n; i++) {
    cin >> x[i];
    cin >> y[i];
  }
  for (int i = 0; i < n; i++) {
    a[x[i]][y[i]] = 1;
  }
  int ans = 0;
  for (int i1 = 1; i1 <= c; i1++) {
    for (int j1 = i1; j1 <= c; j1++) {
      for (int i = 1; i <= r; i++) {
        for (int j = i; j <= r; j++) {
          int x = 0;
          for (int g = i; g <= j; g++) {
            for (int u = i1; u <= j1; u++) {
              if (a[g][u] == 1) {
                x++;
              }
            }
          }
          if (x >= k) {
            ans++;
          }
        }
      }
    }
  }
  cout << ans;
}
