e=[]
a=[]
p=[]
def xiangling(c):
    for d in c:
        if d in e:
           a.append(d)
        e.append(d)
    for j in c:
        if j not in a:
            print(j)
            break
        else:
            p.append(j)
    if e==p:
        print('no')
c=input()
c=c.lower()
xiangling(c)