s = input()
flag = 0
dict1 = {}
for x in s:
    if x in dict1:
        dict1[x] = dict1[x] + 1
    else:
        dict1[x] = 1
for i,j in dict1.items():
    if j == 1:
        flag = 1
        print(i)
        break
if flag == 0:
    print("no")