this is my program:
program df(input,output);
var
ai:array[1..3000] of integer;
aj:array[1..3000] of real;
c:array[1..3000] of real;
i,j,n,m,k:integer;
a0,an1,a1,d:real;
procedure p(i,k:integer;d:real);
begin
 if i=n+2 then begin a1:=(an1-d)/k; writeln(a1:4:2); end
 else
  begin
   ai[i]:=2*k-ai[i-2]; k:=ai[i];
   aj[i]:=2*d+2*c[i-1]-aj[i-2]; d:=aj[i];
   p(i+1,k,d);
  end;
end;
begin
 readln(n);
 readln(a0);
 readln(an1);
 for i:=1 to n do readln(c[i]);
 if n=0 then begin writeln(an1:4:2); halt; end;
 if n=1 then begin writeln(((a0+an1)/2-c[1]):4:2); halt; end;
 k:=2;d:=2*c[1]-a0;
 ai[1]:=1; aj[1]:=0;
 p(3,k,d);
end.
why i got wa???

3
0
4
0
0
0

answer=1
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The sequence is: 0 1 2 3 4

thank you very much.(by geniushjs)this is my E-mail:geniushjs@tom.com