#include<cstdio>
int a[5001],i,j,k,m,n,b[5001],c[5001],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[++t]=a[l],s=l;
for(i=l+1;i<=n;i++)
if(a[i]>a[l]&&b[i]==m-1)
       c[++t]=a[i],f=l=i,m--;
for(k=0,i=s;i<=f;i++)
for(j=1;j<=t;j++)
if(a[i]==c[j])
       k++;
printf("%d",(k==f-s+1)?(f-s+1):-1);
return 0;}