I think this problem is simple but I got WA#14.

var w,d:real;
    l:real;
    lx,ly,x0,y0,x1,y1:real;
    ch:char;
begin
        readln(w,d);
        readln(x0,y0);
        readln(x1,y1);
        lx:=x0-x1;
        ly:=y0-y1;
        while not eof do
        begin
                read(ch);
                case upcase(ch) of
                'L':lx:=lx+2*x1;
                'R':lx:=lx+2*(w-x1);
                'F':ly:=ly+2*y1;
                'B':ly:=ly+2*(d-y1);
                end;
        end;
        l:=sqrt(lx*lx+ly*ly);
        write(l:0:4);
end.

Oh AC now! Very stupid bug:)