#nullable enable

int n;
long k;
{
    var input = Console.ReadLine()!.Split(' ');
    n = int.Parse(input[0]);
    k = long.Parse(input[1]);
}
var s = Console.ReadLine()!;
int C(int j, int l)
{
    var c1 = s[j] - '0';
    if (c1 == 0) return -1;
    if (l == 1) return c1;
    if (c1 >= 3 || j + 1 >= n) return -1;
    var c2 = c1 * 10 + s[j + 1] - '0';
    if (c2 > 26) return -1;
    return c2;
}
var f = new List<string>[n];
List<string> F(int j)
{
    if (j == n) return new(){ "" };
    var res = new List<string>();
    if (f[j] != null) return f[j];
    for (var l = 1; l <= 2; l++)
    {
        var ci = C(j, l);
        if (ci < 0) continue;
        var c = (char)(ci - 1 + 'a');
        var next = F(j + l);
        foreach (var v in next) res.Add(c + v);
    }
    return f[j] = res;
}
var results = F(0);
results.Sort();
Console.WriteLine(results[(int)k - 1]);