#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define ep emplace_back
#define fi first
#define se second
using ll=long long;
using ull=unsigned long long;
#define pi pair<int, int>
#define pll pair<long long,long long>
#ifdef LOCAL
#include"D:/Administrator/Documents/vscode/algo/debug.h"
#else
#define debug(...) 42
#endif

mt19937 mrand(random_device{}());
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signed main(){
#ifndef LOCAL
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
#endif
#ifdef LOCAL
    freopen("D:/Administrator/Documents/vscode/in.txt","r",stdin);
    freopen("D:/Administrator/Documents/vscode/out.txt","w",stdout);
#endif
    int n,d=-1;
    cin>>n;
    vector<int> p(n);
    for(int i=0;i<n;i++){
        cin>>p[i];
        if(i>0){
            int tmp=((p[i]-p[i-1])%n+n)%n;
            if(d==-1||tmp==d){
                d=tmp;
                continue;
            }else{
                cout<<-1;
                return 0;
            }
        }
    }
    //cout<<d<<endl;
    if(p[0]==0){
        if(d==0){
            cout<<0;
        }else if(d==1){
            cout<<1;
        }else{
            cout<<d+1;
        }
    }else{
        if(d==0){
            cout<<n+1;
        }else if(d==1){
            cout<<n+2;
        }else{
            cout<<d+1;
        }
    }
    return 0;
}