var B:array [1..1000] of real;
c:integer;d:real;
begin
writeln('Vvedite N=');read(a);
d:=0;
for c:=1 to n do
if(c mod 2= 0) then d:=d+b[c];
b[n]:=d;writeln(b[n]);readln;readln;
end;
