#include<cstdio>
int a[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),a[0]=-1;
for(i=1;i<=n;i++)
       {scanf("%d",&a[i]);
       if(a[b[m]]<a[i])
              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]])
              b[j]=i;}
for(s=b[1];s<=b[m]-m+1&&r==-1;s++)
       {for(k=0,j=1;b[j]<s&&j<=m;j++);
       for(i=b[j];i<=b[m];i++)
       for(l=j;l<=m;l++)
       if(a[i]==a[b[l]])
              k++;
       if(k==b[m]-b[j]+1&&b[m]-s+1>=m)
              r=b[m]-s+1;}
printf("%d",r);
return 0;}