import std.stdio, std.string, std.conv;
import std.range, std.algorithm, std.array, std.typecons, std.container;
import std.math, std.numeric, core.bitop;



void main() {
    string s;
    scan(s);
    int k;
    scan(k);

    string[] set;
    int n = s.length.to!int;

    foreach (i ; 0 .. n) {
        foreach (j ; i + 1 .. i + k + 1) {
            set ~= s[i .. min(j, $)];
        }
    }

    set.sort();

    debug {
        writeln(set);
    }

    auto ans = set.uniq.drop(k-1).front;
    writeln(ans);
}







































void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}



void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}
