const a1=4.5; d=0.5;
var S:real; n:integer;
begin
readln(n);
S:=(2*a1*n+d*n*(n-1))/(2);
writeln(S);
end.
const a1=4.5; d=0.5;
var S:real; n:integer;
begin
readln(n);
S:=(2*a1*n+d*n*(n-1))/(2);
writeln(S);
end.