#include<stdio.h>
#include<algorithm>
using namespace std;
int n,v[5001],v2[5001];
int sol,lim,lim2,nr;
int main ()
{
    int i,j,st,dr,m;
    freopen("secv.in","r",stdin);
    freopen("secv.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%d",&v[i]);
        v2[i]=v[i];
    }
    sort(v2+1,v2+n+1);
    v2[0]=-1;
    for(i=1;i<=n;i++)
        if(v2[i]!=v2[i-1])
            v2[++nr]=v2[i];
    for(i=1;i<=n;i++)
    {
        st=1;dr=nr;
        while(st<=dr)
        {
            m=(st+dr)/2;
            if(v2[m]<v[i])
                st=m+1;
            else
                if(v2[m]>v[i])
                    dr=m-1;
                else
                {
                    v[i]=m;
                    break;
                }
        }   //while
    }   //for
    lim=0;
    sol=n+1;
    for(i=1;i<=n;i++)
    {
        if(v[i]==lim+1)
            lim++;
        if(lim!=nr)
            continue;
        lim--;
        lim2=v[i];
        for(j=i-1;j>=1 && lim2!=1;j--)
            if(v[j]==lim2-1)
                lim2--;
        if(i-j<sol)
            sol=i-j;
    }
    if(sol==n+1)
        printf("-1\n");
    else
        printf("%d\n",sol);
    return 0;
}