program indexing;
var
A, B, C : AnsiString;
dimA, dimB,i, n, lunghmin :Longint;
uscita:boolean;
begin
(* assign(input, 'input.txt'); reset(input);
assign(output, 'output.txt'); rewrite(output);*)
ReadLn(A);
ReadLn(B);
C := '';
i:=1; uscita:=false;
dimA:=Length(A); dimB:=Length(B);
while (i<=dimA) and (uscita=false) do
begin
writeln(i); writeln(C);
if A[i]=B[i] then begin C:=C+A[i]; i:=i+1; continue; end
else if ord(B[i])-ord(A[i])=1 then begin C:=C+A[i]; i:=i+1; continue; end
else if A[i]<>'z' then begin n:= ord(A[i])+1; C:=C+chr(n); uscita:=true; continue;end
else if A[i]='z' then begin C:=C+'z';i:=i+1; continue;end;
if (i>dimA) then C:=C+'a';
end;
writeln(C) ;
end.
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