#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=1e7+10;
int a[N],p[N],ans[N];
int mod=998244353;
int qmid(int a,int b)
{
    int res=1;
    while(b)
    {
        if(b&1)
            res=res*a%mod;
        a=a*a%mod;
        b>>=1;
    }
    return res;
}
signed main()
{
    int n;
    cin>>n;
    int ans=0;
    map<int,int>mp;
    for(int i=1;i<=n;i++)
    {
        cin>>a[i];
        mp[a[i]]++;
        if(i>1)
        {
            if(a[i]>a[i-1])
                ans=a[i]-a[i-1];
        }
    }
    if(mp.size()!=n)
    {
        cout<<-1<<endl;
        return 0;
    }
    if(a[0]!=0)
        ans++;
    cout<<ans<<endl;
}