import sys
#input = sys.stdin.readline
#input = sys.stdin.buffer.readline #文字列はダメ
#sys.setrecursionlimit(1000000)
import math
import bisect
#import itertools
#import random
from heapq import heapify, heappop, heappush
from collections import defaultdict 
#from collections import deque
import copy #DeepCopy: hoge = [_[:] for _ in hogehoge]
#from functools import lru_cache
#@lru_cache(maxsize=None)
#MOD = pow(10,9) + 7
MOD = 998244353
#dx = [1,0,-1,0]
#dy = [0,1,0,-1]
#dx8 = [1,1,0,-1,-1,-1,0,1]
#dy8 = [0,1,1,1,0,-1,-1,-1]

def main():
    N = int(input())
    A = list(map(int,input().split()))
    A = [a-1 for a in A]

    used = set([])
    total = 1
    for v in range(N):
        if v in used: continue
        # print("V",v)
        cnt = 1
        v = 0
        used.add(v)
        while A[v] != 0:
            cnt += 1
            v = A[v]
            used.add(v)
        # print(total,cnt)
        total = total*cnt//math.gcd(total,cnt)

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

if __name__ == '__main__':
    main()