{pentru i=1,N executa
 Subsir_Maxim[i]=0 {Subsir_Maxim[i] indica lungimea subsirului maximal ce se termina pe pozitia i}
{ pentru j=1,i-1 executa
  daca (S[i]>S[j]) si (Subsir_Maxim[j]>Subsir_Maxim[i]) executa
   Subsir_Maxim[i]=Subsir_Maxim[j]
  sfarsit daca
 sfarsit pentru
 Subsir_Maxim[i]=Subsir_Maxim[i]+1
sfarsit pentru}
program subsir;
var v,smax:array[1..5000] of longint;
    i1,j1,i,j,n,max:integer;
    ok:boolean;
    f:text;
begin
     assign(f,'secv.in');reset(f);readln(f,n);
     for i:=1 to n do read(f,v[i]);
     assign(f,'secv.out');rewrite(f);
     smax[n]:=1;
     for i:=n-1 downto 1 do
         begin
              smax[i]:=0;
              for j:=i+1 to n do
                  if (v[i]<v[j]) and  (smax[j]>smax[i]) then smax[i]:=smax[j];
              smax[i]:=smax[i]+1;
        end;
    max:=-1;
    for i:=1 to n do
        if smax[i]>=max then begin
                                 max:=smax[i];
                                 i1:=i;
                            end;
   for i:=i1+1 to n do
       if (v[i]>v[i1]) and (smax[i]=max-1) then begin
                                               j1:=i;
                                               max:=max-1;
                                               end;
for i:=1 to i1-1 do
         begin
           ok:=true;
           for j:=i1 to j1 do
               if v[i]=v[j] then begin ok:=false;
                                       break;
                                  end;
          if ok=true then begin
                             write(f,'-1');
                             close(f);
                             halt;
                          end;
end;
for i:=j1+1 to n do
         begin
           ok:=true;
           for j:=i1 to j1 do
               if v[i]=v[j] then begin ok:=false;
                                       break;
                                  end;
          if ok=true then begin
                             write(f,'-1');
                             close(f);
                             halt;
                          end;
end;
write(f,j1-i1+1);
close(f);
end.