program indexing;
 
var
    A, B, C : AnsiString;
    dimA, dimB,i, n :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 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;
                    if ((ord(B[i]) - ord(A[i]))=1) and (i<dimB) then begin C:=C+B[i]; writeln(C); exit; end
                                                                else if ((ord(B[i]) - ord(A[i]))=1) and (i=dimB) then 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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