def counting_sort(array,max):
    bucket = [0] * (max+1)

    for i in range(len(array)):
        if array[i] == 'a':
            bucket[0] += 1
        elif array[i] == 'b':
            bucket[1] += 1
        elif array[i] == 'c':
            bucket[2] += 1
        elif array[i] == 'd':
            bucket[3] += 1
        elif array[i] == 'e':
            bucket[4] += 1
        elif array[i] == 'f':
            bucket[5] += 1
        elif array[i] == 'g':
            bucket[6] += 1
        elif array[i] == 'h':
            bucket[7] += 1
        elif array[i] == 'i':
            bucket[8] += 1
        elif array[i] == 'j':
            bucket[9] += 1
        elif array[i] == 'k':
            bucket[10] += 1
        elif array[i] == 'l':
            bucket[11] += 1
        elif array[i] == 'm':
            bucket[12] += 1
        elif array[i] == 'n':
            bucket[13] += 1
        elif array[i] == 'o':
            bucket[14] += 1
        elif array[i] == 'p':
            bucket[15] += 1
        elif array[i] == 'q':
            bucket[16] += 1
        elif array[i] == 'r':
            bucket[17] += 1
        elif array[i] == 's':
            bucket[18] += 1
        elif array[i] == 't':
            bucket[19] += 1
        elif array[i] == 'u':
            bucket[20] += 1
        elif array[i] == 'v':
            bucket[21] += 1
        elif array[i] == 'w':
            bucket[22] += 1
        elif array[i] == 'x':
            bucket[23] += 1
        elif array[i] == 'y':
            bucket[24] += 1
        elif array[i] == 'z':
            bucket[25] += 1
    return (bucket)

def main():
    S = input()
    bucket_list = counting_sort(S, 25)
    for i in range(26):
        if bucket_list[i] == 0:
            if i == 0:
                print('a')
                return
            if i == 1:
                print('b')
                return
            if i == 2:
                print('c')
                return
            if i == 3:
                print('d')
                return
            if i == 4:
                print('e')
                return
            if i == 5:
                print('f')
                return
            if i == 6:
                print('g')
                return
            if i == 7:
                print('h')
                return
            if i == 8:
                print('i')
                return
            if i == 9:
                print('j')
                return
            if i == 10:
                print('k')
                return
            if i == 11:
                print('l')
                return
            if i == 12:
                print('m')
                return
            if i == 13:
                print('n')
                return
            if i == 14:
                print('o')
                return
            if i == 15:
                print('p')
                return
            if i == 16:
                print('q')
                return
            if i == 17:
                print('r')
                return
            if i == 18:
                print('s')
                return
            if i == 19:
                print('t')
                return
            if i == 20:
                print('u')
                return
            if i == 21:
                print('v')
                return
            if i == 22:
                print('w')
                return
            if i == 23:
                print('x')
                return
            if i == 24:
                print('y')
                return
            if i == 25:
                print('z')
                return
    print('None')

main()


Measured memory_bytes: 3400