#include <bits/stdc++.h>
#define Dim 5001
using namespace std;
ifstream f("secv.in");
ofstream g("secv.out");
long Sir[Dim],n,maxim,Dist[Dim],L=2000000000;
long Gasite[Dim],minim=2000000000;

int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
    {
        f>>Sir[i];
        maxim=max(maxim,Sir[i]);
        minim=min(minim,Sir[i]);
        Dist[i]=5005;
    }
    Dist[n]=1; Gasite[n]=1;
    for(long i=n;i>=1;i--)
     for(long j=i+1;j<=n;j++)
    {
        if(Sir[i]<maxim&&Sir[i]+1==Sir[j])
        {
            if(Dist[i]==5005&&Dist[j]==5005)
            Dist[i]=j-i+1;
            else
            if(Dist[i]==5005&&Dist[j]!=5005)
            Dist[i]=Dist[j]+j-i,Gasite[i]=Gasite[j]+1;
            else
            {
                if((Dist[i]<=Dist[j]+j-i)&&Gasite[i]==0)
                Dist[i]=Dist[j]+j-i,Gasite[i]=Gasite[j]+1;
                else
                if((Dist[i]<=Dist[j]+j-i)&&Gasite[i]!=0&&Gasite[j]+1>Gasite[i])
                Dist[i]=Dist[j]+j-i,Gasite[i]=Gasite[j]+1;
            }
        }
        else
        if(Sir[i]==maxim)
        {
            Dist[i]=1,Gasite[i]=1;
            if(Sir[i]<minim) minim=Sir[i];
        }
    }
    for(int i=1;i<=n;i++)
    if(Sir[i]==minim&&Dist[i]<L&&Gasite[i]==maxim-minim+1)  L=Dist[i];
    if(L!=5001) g<<L;
    else g<<-1;
    return 0;
}