#include<bits/stdc++.h>
using namespace std;
#define endl '\n'
#define rep(i,a,n) for(int i=a;i<=n;i++)
#define int long long
#define none 1e9*(-1)
int n,m,k,_;

constexpr int mx=(1<<17);
const int N=1e5+7;

int a[N];

void main2(){
    cin>>n;
    rep(i,1,n){
        cin>>a[i];
    }    
    vector<int>d(n+1,0);
    vector<int>d1(n+1,1);
    d1[n]=1-n;
    int sum=0;
    int mx=-1;
    rep(i,1,n){
        if(i==1)d[i]=a[i]-a[n];
        else d[i]=a[i]-a[i-1];
        sum+=d[i];
        d[i]=abs(d[i]-d1[i]);
        mx=max(d[i],mx);
    }
    if(sum!=0){
        cout<<"-1"; return;
    }
    cout<<mx;
}

signed main(){
    ios::sync_with_stdio(false); cin.tie(0);
    _=1; 
    //cin>>_;
    while(_--){
        main2(); 
        if(_) cout<<endl;
    }
    return 0;
}