number area2(const G& P) {
number A = 0;
for (int i = 0; i < P.size(); ++i)
A += cross(curr(P, i), next(P, i));
return A;
}
/*
polygon area * 2
*/
double area2(const G& P) {
double A = 0;
for (int i = 0; i < P.size(); ++i)
A += cross(curr(P, i), next(P, i));
return A;
}