Public Class Program
  Shared Sub Main()
    Dim ab As Long() = Array.ConvertAll(Of String, Long)(Console.ReadLine().Split(" "), AddressOf Long.Parse)
    Dim a As Long = ab(0)
    Dim b As Long = ab(1)
    Dim k As Double = (a * a - b * b) / (2 * (a - b))
    Dim output As String = If(k = Math.Ceiling(K), k.ToString(), "IMPOSSIBLE")
    Console.WriteLine(output)
  End Sub
End Class