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