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