Const
 max=205;
 inf=99999999;
Var
 i,j,k,n,k1:longint;
 h,opt:array[1..max]of longint;
 t:array[0..32]of longint;
 flag:boolean;
Begin
 assign(input,'ladder.in');
 assign(output,'ladder.out');
 reset(input);
 rewrite(output);
 flag:=true;
 readln(n);
 t[0]:=1;
 for i:=1 to 30 do
  t[i]:=t[i-1] shl 1;
 for i:=1 to n do
  read(h[i]);
 for i:=2 to n do
  if flag then
   begin
    opt[i]:=inf;
    if h[i]=h[i-1]+1 then
     opt[i]:=opt[i-1]+1;
    for j:=1 to i-1 do
     begin
      if j-30>1 then
       k1:=j-30
                else
       k1:=1;
      for k:=k1 to j-1 do
       if ((h[k]+t[j-k]>=h[i])and(opt[i]>opt[j]+j-k+1))then
        opt[i]:=opt[j]+j-k+1;
     end;
    if opt[i]=inf then
     flag:=false;
   end;
 if flag then
  writeln(opt[n])
         else
  writeln(-1);
 close(input);
 close(output);
End.