#include <fstream>
#include<set>
using namespace std;
int n,v[6000],in,i,nr[6000],k,j;
set<int> s;
int main ()
{ifstream f("secv.in");
 ofstream g("secv.out");
f>>n;
for(i=1;i<=n;i++)
{f>>v[i]; s.insert(v[i]);}

while(!s.empty()){
nr[++k]=*(s.begin());
s.erase(s.begin());}

for(i=n-k;i&&in<=k;i--)
for(in=1,j=i;j<=n&&in<=k;j++)
if(v[j]==nr[in]) in++;

if(in==k+1)
g<<j-i-1;
else
g<<-1;
f.close(); g.close();
return 0;
}