from collections import *
class x(Counter, OrderedDict):
  pass
def NonRepeatingChar(s):
   d = x(s)
   print(d)
   flag = 0
   for k, v in d.items():
       if v == 1:
          flag = 1
          print(k)
          break
   if (flag == 0):
       print(-1)

   if __name__ == "__main__":
      for i in range(int(input())):
           input()
           s = input()
           NonRepeatingChar(s)