#include <bits/stdc++.h> //Andrei Alexandru a.k.a Sho
#define ll long long
#define double long double
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#define aint(a) (a).begin(), (a).end()
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define pi pair
#define rc(s) return cout<<s,0
#define endl '\n'
#define mod 1000000007ll
#define PI 3.14159265359
#define INF 1000000005
#define LINF 1000000000000000005ll
#define CODE_START  ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
ll n,a[5005];
pair<ll,ll>dp[5005];
map<ll,ll>m;
vector<ll>v;
int32_t main(){
CODE_START;
ifstream cin("secv.in");
ofstream cout("secv.out");
cin>>n;
for(ll i=1;i<=n;i++)
{
    cin>>a[i];
    v.pb(a[i]);
}
sort(v.begin(),v.end());
ll s1=0;
for(ll i=0;i<v.size();i++)
{
    if(i==0){
        m[v[i]]=++s1;
    }else if(v[i]!=v[i-1]){
    m[v[i]]=++s1;
    }
}
for(ll i=1;i<=n;i++)
{
a[i]=m[a[i]];
}
ll ans=LINF;
for(ll i=1;i<=n;i++)
{
    if(a[i]==1){
        ll val=2;
        ll res=1;
        for(ll j=i+1;j<=n;j++)
        {
            if(val>s1){
                break;
            }
            if(a[j]==val){
                val++;
            }
            res++;
        }
        if(val<=s1){
            continue;
        }
        ans=min(ans,res);
    }
}
if(ans==LINF){
    ans=-1;
}
cout<<ans<<endl;
}