#include <bits/stdc++.h>
int n, m, i, j, nrv, k, x, y, c1, c2, l1, l2, nr, numar, a[20][20];
int main() {
  scanf("%d%d%d%d", &n, &m, &nrv, &k);
  for (i = 1; i <= nrv; i++) {
    scanf("%d%d", &x, &y);
    a[x][y] = 1;
  }
  for (l1 = 1; l1 <= n; l1++) {
    for (c1 = 1; c1 <= m; c1++) {
      for (l2 = l1; l2 <= n; l2++) {
        for (c2 = c1; c2 <= m; c2++) {
          nr = 0;
          for (i = l1; i <= l2; i++) {
            for (j = c1; j <= c2; j++) {
              if (a[i][j] == 1) nr++;
            }
          }
          if (nr >= k) numar++;
        }
      }
    }
  }
  printf("%d", numar);
  return 0;
}
