looking for 13:
  window [1..8] mid=4 A[mid]=7
    window [5..8] mid=6 A[mid]=11
      window [7..8] mid=7 A[mid]=13
result: 7

looking for 6:
  window [1..8] mid=4 A[mid]=7
    window [1..3] mid=2 A[mid]=3
      window [3..3] mid=3 A[mid]=5
        window [4..3] is empty -> NIL
result: NIL
