uses crt,math;
type int = longint;
var a,b,s1,s2:string;
l,i,m,j:int;
begin
clrscr;
readln(a);
readln(b);
l:=length(a);
m:=min(l,length(b));
s1:=a[l];
s2:=b[1];
dec(l);i:=2;
for j:=m downto 1 do
begin
if s1 <> s2 then
begin
s1:=a[l] + s1;
s2:=s2 + b[i];
dec(l);inc(i);
end
else
break;
end;
if s1 <> s2 then writeln('khong co')
else writeln(s1);
readln;
end.
