import sys
sys.setrecursionlimit(10**6)
from collections import defaultdict

def dfs(v, t, f, used):
    if v==t:
        return f
    
    used[v] = True
    
    for nv, cap in edges[v].items():
        if not used[nv] and cap>0:
            d = dfs(nv, t, min(f, cap), used)
            
            if d>0:
                edges[v][nv] -= d
                edges[nv][v] += d
                
                return d
    
    return 0

def max_flow(s, t):
    flow = 0
    
    while True:
        used = [False]*(2*N+2)
        f = dfs(s, t, 10**18, used)
        
        if f==0:
            return flow
        
        flow += f
        
N = int(input())
A = [int(input()) for _ in range(N)]
edges = defaultdict(dict)

for i in range(1, N+1):
    edges[0][i] = 1
    edges[i][0] = 0

for i in range(1, N+1):
    for j in range(N+1, 2*N+1):
        if j!=A[i-1]+N+1:
            edges[i][j] = 1
            edges[j][i] = 0

for i in range(N+1, 2*N+1):
    edges[i][2*N+1] = 1
    edges[2*N+1][i] = 0

f = max_flow(0, 2*N+1)

if f<N:
    print(-1)
    exit()

for i in range(1, N+1):
    for j in range(N+1, 2*N+1):
        if j in edges[i] and edges[i][j]==0:
            print(j-N-1)
            break