Public Class Program
  Shared Sub Main()
    Dim n As Long = Cin.Lng()
    Dim a As Long() = Cin.LngArr()
    Dim ans As Long = 0
    Dim i As Long = 1
    For j As Long = 0 To n - 1
      If Not a(j) = i Then
        ans += 1
      Else
        i += 1
      End If
    Next
    If ans = n Then ans = -1
    Console.WriteLine(ans.ToString())
  End Sub
End Class

Public Class Cin
  Public Shared Function Str() As String
    Return Console.ReadLine().Trim()
  End Function
  Public Shared Function Int() As Integer
    Return Integer.Parse(Str())
  End Function
  Public Shared Function Lng() As Long
    Return Long.Parse(Str())
  End Function
  Public Shared Function Dbl() As Double
    Return Double.Parse(Str())
  End Function
  Public Shared Function StrArr(Optional separator As String = " ") As String()
    Return Str().Split(separator)
  End Function
  Public Shared Function IntArr(Optional separator As String = " ") As Integer()
    Return Array.ConvertAll(Of String, Integer)(StrArr(separator), AddressOf Integer.Parse)
  End Function
  Public Shared Function LngArr(Optional separator As String = " ") As Long()
    Return Array.ConvertAll(Of String, Long)(StrArr(separator), AddressOf Long.Parse)
  End Function
  Public Shared Function DblArr(Optional separator As String = " ") As Double()
    Return Array.ConvertAll(Of String, Double)(StrArr(separator), AddressOf Double.Parse)
  End Function
End Class