#include<stdio.h>
#define N 5001
long a[N];
int b[N],p[N],i,j,k,c,m,n,u,v,l; 
int main()
{freopen("secv.in","r",stdin);
freopen("secv.out","w",stdout);
scanf("%d\n",&n);
a[0]=b[0]=m=0;
for(i=1;i<=n;i++)
       {scanf("%ld",&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>0)
                     p[i]=b[j-1];
              b[j]=i;}}
for(u=m,v=b[m];u--;v=p[v])
       b[u]=v;
if(m==0)
       printf("-1\n");
else
       {l=0;
       for(j=b[0];j<=b[m-1];j++)
              {k=0;
              for(i=0;i<=m;i++)
              if(a[b[i]]==a[j]&&k==0)
                       {l++;
                       k=1;}}
       if(l==b[m-1]-b[0]+1)
              printf("%d\n",l);
       else
              printf("-1\n");}
fclose(stdin);
fclose(stdout);
return 0;}