#Dinic法で最大流を求める
#deque のimport が必要
#逆辺追加しなきゃいけないから、
#グラフの構成はadd_edgeで行う
#最大流は　flow    メソッドで

from collections import deque
class Dinic:
    def __init__(self,N):
        self.N = N
        self.G = [[] for _ in range(N)]
        self.level = None
        self.progress = None
    def add_edge(self,fr,to,cap):
        forward = [to,cap,None]
        forward[2] = backward = [fr,0,forward]
        self.G[fr].append(forward)
        self.G[to].append(backward)
    def add_multi_edge(self,v1,v2,cap1,cap2):
        edge1 = [v2,cap1,None]
        edge1[2] = edge2 = [v1,cap2,edge1]
        self.G[v1].append(edge1)
        self.G[v2].append(edge2)
    def bfs(self,s,t):
        self.level = level = [None] * self.N
        q = deque([s])
        level[s] = 0
        G = self.G
        while q:
            v = q.popleft()
            lv = level[v] + 1
            for w,cap,_ in G[v]:
                if cap and level[w] is None:
                    level[w] = lv
                    q.append(w)
        return level[t] is not None
    def dfs(self,v,t,f):
        if v == t:return f
        level = self.level
        Gv = self.G[v]
        for i in range(self.progress[v],len(Gv)):
            self.progress[v] = i
            w,cap,rev = e = Gv[i]
            if cap and level[v] < level[w]:
                d = self.dfs(w,t,min(f,cap))
                if d:
                    e[1] -= d
                    rev[1] += d
                    return d
        return 0
    def flow(self,s,t,):
        flow = 0
        inf = 1 << 30
        G = self.G
        while self.bfs(s,t):
            self.progress = [0] * self.N
            f = inf
            while f:
                f = self.dfs(s,t,inf)
                flow += f
        return flow
    def min_cut(self,s):
        #最小カットを実現する頂点の分割を与える
        #True  なら　source側
        #False なら　sink側
        visited = [False for i in range(self.N)]
        q = deque([s])
        while q:
            now = q.popleft()
            visited[now] = True
            for to,cap,_ in self.G[now]:
                if cap and not visited[to]:
                    visited[to] = True
                    q.append(to)
        return visited
    
N = int(input())

import sys
if N == 1:
    a = int(input())
    if a == 0:
        print(-1)
        exit()
    else:
        print(0)
        exit()

dinic = Dinic(N + N + 2)
T = N + N + 1
for i in range(N):
    a = int(input())
    for j in range(N):
        if j == a:continue
        dinic.add_edge(i+1,j+1+N,1)
for i in range(N):
    dinic.add_edge(0,i+1,1)
for j in range(N):
    dinic.add_edge(j+1+N,T,1)
a = dinic.flow(0,T)
if a < N:
    print(-1)
    exit()
for i in range(N):
    for to,cap,rev in dinic.G[i+1]:
        if N < to < T and cap == 0:
            print(to - N - 1)
            break