Here is my code.
Somebody please help me to find a mistake!


Program n1048;
Var
   A:Array [1..500000] Of Byte;
   n,i:Longint;
   k,x1,y1,x2,y2:byte;
Begin
     Readln(N);
     If (n mod 2)<>0 Then
     Begin
          Readln(x1,y1);
          a[1]:=x1+y1;
          For i:=1 To (n div 2)
          Do Begin
             Readln(X1,Y1);Readln(x2,y2);
             a[i+1]:=x1*10+x2+y1*10+y2;
          End;
          k:=0;
          For I:=((n div 2) +1) DOWnto 1
          Do Begin
             y1:=a[i];
             a[i]:=(y1+k) mod 100;
             k:=(y1+k) div 100;
          End;
          Write(a[1]);
          For i:=2 To ((n div 2)+1) Do
              If a[i]<>0 Then Write(a[i])
              Else Write(a[i],a[i]);
     End;
     If (n mod 2)=0 Then
     Begin
          For i:=1 To (n div 2)
          Do Begin
             Readln(X1,Y1);Readln(x2,y2);
             a[i]:=x1*10+x2+y1*10+y2;
          End;
          k:=0;
          For I:=(n div 2) DOWnto 1
          Do Begin
             y1:=a[i];
             a[i]:=(y1+k) mod 100;
             k:=(y1+k) div 100;
          End;
          For i:=1 To (n div 2) Do
              If a[i]<>0 Then Write(a[i])
                 Else Write(a[i],a[i]);
     End;
     Readln;
End.

Here is a Test
4
9 0
9 0
9 0
1 9
The Right Answer is 10000
Your Answer is 0000
Do You Test You Program Before Submit It?:)

They say the length of a sum never exceeds N, so your test is wrong. But still, take that in mind.