N = gets.to_i
A = gets.split.map(&:to_i)

graph = Array.new(N) { [] }
N.times do |u|
    v = A[u] - 1
    graph[u] << v
    graph[v] << u
end

count = 0
visited = [false] * N
N.times do |s|
    next if visited[s]
    visited[s] = true
    count += 1
    stack = [s]
    while (u = stack.pop)
        count += 1
        graph[u].each do |v|
            next if visited[v]
            visited[v] = true
            stack << v
        end
    end
end

puts count.even? ? 1 : -1