10432 - Polygon Inside A Circle

#include<bits/stdc++.h>
using namespace std;
#define PI acos(-1.0)
long double getArea(long double a, long double b, long double c)
{
    long double s = (a+b+c)/2.0;
    long double area = (s*(s-a)*(s-b)*(s-c));
    area = sqrt(area);
    return area;
}
long double degTOrad(long double deg)
{
    long double ret = (deg*PI)/180.0;
    return ret;
}
long double getThird(long double a, long double b, long double C)
{
    long double ret =  (a*a) + (b*b) - (2.0*a*b*(cos(C)));
    ret = sqrt(ret);
    return ret;
}
int main()
{
    long double r,n;
    while(cin>>r>>n){
        long double theta = (360.0)/n;
        theta = degTOrad(theta);
        long double c = getThird(r,r,theta);
        long double area = getArea(r,r,c);
        area = area*n;
        long long ans = area*1000;
        long long tem = area*10000;
        if((tem%10)>=5)ans++;
        area = ans/1000.0;
        cout<<fixed<<setprecision(3)<<area<<endl;
    }
    return 0;
}