This is my SOLUTION.
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var           S, Sr1, sr2: string;
               i, i1, l, N, sum, sl, code, W: integer;
Procedure One_summ1(var sum:integer);
  Begin    Sum:=0;
  For l:=1 to length(sr1) do
    begin
          val(sr1[l],sl,code);
          Sum:=sum+l*Sl;     end;   end;

Begin
  Readln(N);
  Repeat
    Readln(S);
    Val(S,W,code);
    If W=0 then begin
                      for i:=1 to Length(S) do Write('0');
                      continue;
                      end;
    If Length(s)=N+1 then
         begin
         For i1:=1 to length(S) do
           begin
           sr1:=s;
           Delete(sr1,i1,1);
           One_summ1(sum);
             If ((sum)mod(n+1)=0) then
               begin
               Writeln(Sr1);
               Break;     end;   end;    end;

      If Length(s)=N then
         begin
         For i1:=1 to length(S) do
           begin
           sr1:=s;
           If sr1[i1]='1' then sr1[i1]:='0'
                          else continue;
           One_summ1(sum);
             If ((sum)mod(n+1)=0) then
               begin
               Writeln(Sr1);
               Break;     end;    end;    end;

      If Length(s)=N-1 then
        Begin
        For i1:=1 to length(S)+1 do
          begin
          sr1:=s;     sr2:=s;
          Insert('1',sr1,i1); Insert('0',sr2,i1);
          One_summ1(sum);
             If ((sum)mod(n+1)=0) then
             begin
               Writeln(Sr1);
               Break;
             end;
          sr1:=sr2;
          One_summ1(sum);
             If ((sum)mod(n+1)=0) then
             begin
               Writeln(Sr1);
               Break;
               end;       end;      end;
    until EOF;
End.
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I suppose, the output is INCORRECT.
              PLEASE, HELP ME OR WRITE SOME TEST!
                                Thank you.

you shoudln't use string; because the number 'n' is >> 4<= n <=1000,
but string is only 255 long.