#!/usr/bin/env python3

import sys


def parse_input():
    line = sys.stdin.readline().strip()
    if not line:
        return []
    codes = list(map(int, line.split()))
    return codes


def decode(codes):
    phrases = {i: chr(ord('a') + i) for i in range(26)}
    next_code = 26

    result = []
    prev_phrase = ""

    for code in codes:
        if code in phrases:
            phrase = phrases[code]
        elif code == next_code and prev_phrase:
            phrase = prev_phrase + prev_phrase[0]
        else:
            phrase = ""

        result.append(phrase)

        if prev_phrase and phrase:
            new_phrase = prev_phrase + phrase[0]
            phrases[next_code] = new_phrase
            next_code += 1

        prev_phrase = phrase

    return ''.join(result)


def main():
    codes = parse_input()
    print(decode(codes))


if __name__ == "__main__":
    main()