This is my program:
var
  a,c:array [0..3001] of real;
  n:integer;

procedure init;
var
  i:integer;
begin
  read(n);
  read(a[0],a[n+1]);
  for i:=1 to n do
    read(c[i]);
end;

function work(i:integer):real;
begin
  if i=0 then work:=a[0] else
  if i=n+1 then work:=a[n+1] else
  work:=(work(i-1)+work(i+1))/2-c[i];
end;

procedure print;
begin
  writeln(work(1):0:2);
end;

begin
  init;
  print;
end.

Your algoritmh not correct again. It very long. This problem solve with other algoritmh.
Used that a[0] = 2*a[1] + 2*c[1] - a[2] = 3*a[2] - 2*a[3] + 2*c[1] + 4*c[2], etc.