n, k = map(int, input().split())
t = list(map(int, input()))
cnt = [-1] * (n + 1)


def solve(i):
    if cnt[i] != -1:
        return cnt[i]
    if i == n:
        cnt[i] = 1
        return 1
    if t[i] == 0:
        cnt[i] = 0
        return 0

    cnt[i] = solve(i + 1)
    if i != n - 1 and t[i] * 10 + t[i + 1] <= 26:
        cnt[i] += solve(i + 2)
    return min(10**18 + 2525, cnt[i])


solve(0)

p = 0
k -= 1
ans = []

while p < n:
    if k < cnt[p + 1]:
        ans.append(chr(96 + t[p]))
        p += 1
    else:
        c = t[p] * 10 + t[p + 1]
        ans.append(chr(96 + c))
        k -= cnt[p + 1]
        p += 2
assert k == 0
print(*ans, sep="")