#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int r, c, n, k, x, y, i;
  cin >> r >> c >> n >> k;
  int a[r + 1][c + 1];
  memset(a, 0, sizeof(a));
  for (i = 0; i < n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  int j, ii, jj, l, o, ans = 0, out = 0;
  for (i = 1; i <= r; i++) {
    for (ii = i; ii <= r; ii++) {
      for (j = 1; j <= c; j++) {
        for (jj = j; jj <= c; jj++) {
          ans = 0;
          for (l = i; l <= ii; l++) {
            for (o = j; o <= jj; o++) {
              if (a[l][o] == 1) {
                ans++;
              }
            }
          }
          if (ans >= k) {
            out++;
          }
        }
      }
    }
  }
  cout << out;
}
