#include<cstdio>
int a[5001],p[5001],i,j,k,m,n,b[5001],c,r=-1,s,l; 
int main()
{freopen("secv.in","r",stdin),freopen("secv.out","w",stdout);
scanf("%d",&n);
for(i=1;i<=n;i++)
       {scanf("%d",&a[i]);
       if(a[b[m]]<a[i])
              p[i]=b[m],b[++m]=i;
       for(j=0,k=m;j<k;)
              {c=(j+k)/2;
              if(a[b[c]]<a[i])
                        j=c+1;
              else
                        k=c;}
       if(a[i]<a[b[j]])
              {if(j)
                        p[i]=b[j-1];
              b[j]=i;}}
for(i=m,j=b[m];i--;j=p[j])
       b[i]=j;
for(s=1;s<=n;s++)
       {for(k=j=0;b[j]<s&&j<m;j++);
       for(i=b[j];i<=b[m-1];i++)
       for(l=j;l<m;l++)
       if(a[i]==a[b[l]])
              k++;
       if(k==b[m-1]-b[j]+1)
              r=b[m-1]-s+1;
       else
              if(r!=-1)
                    break;}
printf("%d",r);
return 0;}