#include<bits/stdc++.h>
using namespace std;
#define endl "\n"
#define debug(x) cout<<#x<<": "<<x<<endl
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
using ll=long long;
using ull=unsigned long long;
const int N=200010;
int a[N],n;
int dis(int x,int y){
    if(x>=y){
        return x-y;
    }else{
        return n+x-y;
    }
}
void solve(){
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>a[i];
    }
    int ans=0,d=dis(a[1],a[0]);
    for(int i=0;i<n;i++){
        if(dis(a[(i+1)%n],a[i])!=d){
            cout<<-1<<endl;
            return;
        }
    }
    if(d==0){
        if(a[0]==0){
            cout<<0<<endl;
        }else{
            cout<<-1<<endl;
        }
        return;
    }
    ans+=d-1;
    if(a[0]!=0){
        ans+=2;
    }
    cout<<ans<<endl;
}
signed main(){
    ios::sync_with_stdio(0);
    cout.tie(0);
    cin.tie(0);
    int t=1;
    while(t--){
        solve();
    }
}
/*
贡献法
正难则反
数小状压
关系连边
拆位
广义单调性
最长转最短
*/