var
s:String;
i,j,k:Longint;
a,b,c,d,e:String[5];
begin
readln(s);
read(k);
a:='zzzzz';
b:=a;
c:=b;
d:=c;
e:=d;
for i:=1 to length(s)do begin
for j:=1 to k do begin
if i+j-1>length(s)then break;
if a>copy(s,i,j)then begin
e:=d;
d:=c;
c:=b;
b:=a;
a:=copy(s,i,j);
end else if a<>copy(s,i,j)then begin
if b>copy(s,i,j)then begin
e:=d;
d:=c;
c:=b;
b:=copy(s,i,j);
end else if b<>copy(s,i,j)then begin
if c>copy(s,i,j)then begin
e:=d;
d:=c;
c:=copy(s,i,j);
end else if c<>copy(s,i,j)then begin
if d>copy(s,i,j)then begin
e:=d;
d:=copy(s,i,j);
end else if d<>copy(s,i,j)then begin
if e>copy(s,i,j)then begin
e:=copy(s,i,j);
end;
end;
end;
end;
end;
end;
end;
if k=1 then writeln(a)else if k=2 then writeln(b)else if k=3 then writeln(c)else if k=4 then writeln(d)else writeln(e);
end.