import sys
sys.setrecursionlimit(10**6)

moji=set()
s=input()
n=len(s)

def  cal(ind):
    if ''.join(t) in moji:
        return
    if ind == len(s):
        moji.add(''.join(t))
        return
    moji.add(''.join(t))
    for i in range(ind,len(s)):
        t.append(s[i])
        cal(i+1)
        t.pop()
        
for i in range(len(s)):
    t=[s[i]]
    cal(i+1)

def  cal(ind):
    if len(t) > n+1:
        return
    if ''.join(t) not in moji:
        print(''.join(t))
        exit()
    for i in range(ind,123):
        t.append(chr(i))
        cal(i)
        t.pop()


for i in range(40):
    t=[chr(97+i)]
    cal(97+i)