#include <bits/stdc++.h>
using namespace std;
int a[15][15];
int b[15][15];
int main() {
  int n, m, k, l;
  cin >> n >> m >> k >> l;
  for (int i = 1; i <= k; i++) {
    int x, y;
    cin >> x >> y;
    a[x][y] = 1;
  }
  b[1][1] = a[1][1];
  for (int i = 2; i <= m; i++) b[1][i] = b[1][i - 1] + a[1][i];
  for (int i = 2; i <= n; i++)
    for (int j = 1; j <= m; j++) {
      b[i][j] = b[i - 1][j] + b[i][j - 1] - b[i - 1][j - 1] + a[i][j];
    }
  long long ans = 0;
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= m; j++)
      for (int i1 = 1; i1 <= n; i1++)
        for (int j1 = 1; j1 <= m; j1++) {
          if (i1 > i || (i1 == i && j <= j1)) {
            int l1 = b[i1][j1] - b[i1][j - 1] - b[i - 1][j1] + b[i - 1][j - 1];
            if (l1 >= l) ans++;
          }
        }
  cout << ans;
  return 0;
}
