Pascal :

var n,m,i,j,head:longint;
    str1,str2:ansistring;
 
procedure qsort(l,r:longint);
var i,j:longint;
    t,mid:char;
begin
  i:=l; j:=r; mid:=str2[(i+j) div 2];
  repeat
    while str2[i]>mid do inc(i);
    while str2[j]<mid do dec(j);
    if i<=j then
    begin
      t:=str2[i]; str2[i]:=str2[j]; str2[j]:=t;
      inc(i); dec(j);
    end;
  until i>j;
  if l<j then qsort(l,j);
  if i<r then qsort(i,r);
end;
 
begin
  readln(n,m);
  readln(str1);
  readln(str2);
  qsort(1,m);
  head:=1;
  for i:=1 to n do
  begin
    if (str2[head]>str1[i]) and (head<=m) then
    begin
      str1[i]:=str2[head];
      inc(head);
    end;
  end;
  writeln(str1);
end.