더블릿|dovelet - 정다각형 면적/rpoly

문제

#include <iostream>
#include <cmath>
 
using namespace std;
 
int main()
{
    const double PI = 3.14159265359;
    double r;
    double n;
    cin >> r >> n;
    double rad = (360 / n) * (PI / 180);
    cout.precision(3);
    cout << fixed << (0.5 * r *r * sin(rad) * n) << endl;
 
    return 0;
}