#include<cstdio>
#include<algorithm>
using namespace std;
int v[5001],v2[5001];
int main ()
{freopen ("secv.in","r",stdin);
freopen ("secv.out","w",stdout);
int n,i,j,in=0,sf=0,x,pp;
scanf("%d",&n);
for(i=1;i<=n;i++)
    {scanf("%d",&v[i]);
    v2[i]=v[i];
    }
sort(v2+1,v2+n+1);
j=1;
i=1;
pp=0;
for(j=1;j<=n&&pp==0;j++)
    if(v[j]==v2[i])
        {in=j;
        if(v2[i]==v2[n])
            {printf("0");
            return 0;
            }
        i++;
        while(v2[i]==v2[i-1])
            i++;
        pp=1;
        }
for(;i<=n;i++)
    {pp=0;
    for(;j<=n&&pp==0;j++)
        if(v[j]==v2[i])
            {if(v2[i]==v2[n])
                {printf("%d",j-in+1);
                return 0;
                }
            i++;
            while(v2[i]==v2[i-1])
                i++;
            pp=1;
            }
    }
printf("-1");
return 0;
}