a = list(input())
c = 0
for i in range(len(a) - 1, -1, -1):
    if a[i] != 'z':
        b = ord(a[i])
        b += 1
        a[i] = chr(b)
        break
    else:
        c += 1
if c == len(a):
    print('NO SOLUTION')

elif c != 0:
    for i in range(len(a) - 1, len(a) - 1 - c, -1):
        a[i] = 'a'
    print(''.join(a))
else:
    print(''.join(a))