program a1047;
var
c:array[1..3000]of longint;
n,i:longint;
a0,an1,a1,s:real;
begin
    read(n);
    read(a0);
    read(an1);
    for i:=1 to n do read(c[i]);
    s:=0;
    for i:=1 to n do
        s:=s+i*c[n-i+1];
    a1:=a0+(an1-a0-2*s)/(n+1);
    writeln(a1:0:2);
end.

> program a1047;
> var
> c:array[1..3000]of longint;
> n,i:longint;
> a0,an1,a1,s:real;
> begin
>     read(n);
>     read(a0);
>     read(an1);
>     for i:=1 to n do read(c[i]);
>     s:=0;
>     for i:=1 to n do
>         s:=s+i*c[n-i+1];

This string must look like that:
                s:=s+c[i]*(n-1+1);






>     a1:=a0+(an1-a0-2*s)/(n+1);

      ..and this one like that a1:=(an1+n*a0-2*s)/(n+1);




>     writeln(a1:0:2);
> end.