#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[1]=1-n;
    int sum=0;
    int mx=-1;
    int cnt=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];
        mx=max(mx,abs(d[i]));
    }
    rep(i,1,n){
        if(mx+abs(d[i])!=n) cnt=0;
    if(sum!=0){
        cout<<"-1"; return;
    }
    if(cnt)cout<<mx;
    else cout<<n+1-mx;
}

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

answer.code: In function ‘void main2()’:
answer.code:40:12: warning: empty parentheses were disambiguated as a function declaration [-Wvexing-parse]
   40 | signed main(){
      |            ^~
answer.code:40:12: note: remove parentheses to default-initialize a variable
   40 | signed main(){
      |            ^~
      |            --
answer.code:40:12: note: or replace parentheses with braces to value-initialize a variable
answer.code:40:14: error: a function-definition is not allowed here before ‘{’ token
   40 | signed main(){
      |              ^
answer.code:49:2: error: expected ‘}’ at end of input
   49 | }
      |  ^
answer.code:14:13: note: to match this ‘{’
   14 | void main2(){
      |             ^