#include<iostream>
#include<map>
#include<set>
#include<cmath>
#include<vector>
#include<queue>
#include<deque>
#include<string>
#include<cstdlib>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<math.h>
#include<stack>
#include<stdint.h>
#include<bitset>
#include<fstream>
#include<time.h>
#define IOS cin.tie(0) -> sync_with_stdio(false), cout.tie(0);
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
const int N=1e6+5;
const int M=1e6+5;
const int inf=0x3f;
#define lc p<<1
#define rc p<<1|1
#define endl "\n"
double pai = acos(-1);
ll mod=1e9+7;
inline ll read()
{
    int x=0,f=1;char ch=getchar();
    while (ch<'0'||ch>'9'){if (ch=='-') f=-1;ch=getchar();}
    while (ch>='0'&&ch<='9'){x=x*10+ch-48;ch=getchar();}
    return x*f;
}
//__int128 read_128()//c++20(64)可用
//{
//    __int128 x=0,f=1;char ch = getchar();
//    while(ch < '0' || ch > '9'){if(ch == '-') f=-1;ch=getchar();}
//    while(ch >= '0' && ch <= '9'){x = x * 10 + ch - '0';ch=getchar();}
//    return x*f;
//}
//void print_128(__int128 x)
//{
//    if(x<0){putchar('-');x=-x;}
//    if(x>9)print_128(x/10);
//    putchar(x % 10 + '0');
//}
int idx=1,nxt[M*2],dian[M*2],h[N];
ll quan[M*2];
//ll rong[M*2];
void add(int a,int b,ll c)
{
    dian[++idx]=b;
    quan[idx]=c;
    //rong[idx]=c;
    nxt[idx]=h[a];
    h[a]=idx;
}
//ll ksm(ll a,ll b){ll sum=1;while(b){if(b&1){sum*=a;}a*=a;b>>=1;}return sum;}
//ll ksm_mod(ll a,ll b,ll p){ll sum=1;while(b){if(b&1){sum*=a;sum%=p;}a*=a;a%=p;b>>=1;}return sum%p;}
//ll gsc(ll a,ll b,ll p){ll sum=0;while(b){if(b&1){sum+=a;sum%=p;}a=a*2;a%=p;b/=2;}return sum%p;}


int n;
ll a[N];
ll b[N];
int main()
{
    cin >> n;
    ll ans=0;
    ll mk=-1;
    for(int i=0;i<n;i++)
    {
        cin >> a[i];
        if(a[i]==0)
        {
            ans++;
        }
        if(a[i]==0&&mk==-1)
        {
            mk=i;
        }
    }
    if(ans==n)
    {
        cout << "0\n";
        return 0;
    }
    b[0]=a[0];
    ll x=a[1]-a[0];
    
    x=(x%n+n)%n;
    for(ll i=1;i<n;i++)
    {
        b[i]=i*x+b[0];
        b[i]=(b[i]%n+n)%n;
    }
    bool fl=0;
    for(int i=0;i<n;i++)
    {
        if(a[i]!=b[i])
        {
            fl=1;
            break;
        }
    }
    if(fl||mk==-1)
    {
        cout << "-1\n";
        return 0;
    }
    if(a[0]==0)
    {
        cout << x << endl;
    }
    else
    {
        cout << x+1 << endl;
    }
    return 0;
}