var n, i, k, sum,temp:integer;
begin
   readln(temp);
   k:=1;
   sum:=temp;
   while(temp <> 0) do begin
      readln(temp);
      if (temp<>0) and (temp>=10) and (temp<=99) then begin
         k:=k+1;
         sum:=sum+temp;
      end;
   end;

   if (k = 0) then
      writeln('NO')
   else
      writeln(sum/k);
end.