"""

"""

import sys

def uf_find(n,p):

    ufl = []

    while p[n] != n:
        ufl.append(n)
        n = p[n]

    for i in ufl:
        p[i] = n

    return n


def uf_union(a,b,p,rank):

    ap = uf_find(a,p)
    bp = uf_find(b,p)

    if ap == bp:
        return True
    else:

        if rank[ap] > rank[bp]:
            p[bp] = ap
        elif rank[ap] < rank[bp]:
            p[ap] = bp
        else:
            p[bp] = ap
            rank[ap] += 1

        return False

N = int(input())

A = list(map(int,input().split()))

p = [i for i in range(N)]
rank = [0] * N

for i in range(N):

    uf_union(i,A[i]-1,p,rank)

ans = N
for i in range(N):
    if uf_find(i,p) == i:
        ans -= 1

if ans % 2 == 1:
    print (-1)
else:
    print (1)