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