from collections import Counter
def run(seq: str):
d = dict(Counter(seq))
for k,v in d.items():
if v == 1:
return k
return -1
seq = input().strip()
print(run(seq))
from collections import Counter
def run(seq: str):
d = dict(Counter(seq))
for k,v in d.items():
if v == 1:
return k
return -1
seq = input().strip()
print(run(seq))