N = int(input())
S = list(map(lambda x: int(x)-1, input().split()))

xs = S[:]
cnt = 0
for i in range(N):
    if xs[i] == i: continue

    for j in range(i+1, N):
        if xs[j] == i:
            xs[i], xs[j] = xs[j], xs[i]
            cnt += 1
            break

if cnt % 2 == 0:
    print(1)
else:
    print(-1)