here's my code. I've tried many testes but i've passed them\

#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;

double f(double r, double a) {
        double rad=(a*4*atan((double)1))/180;
        double da=sqrt(
        2*r*r-2*r*r*cos(rad)
        );
        return da;
}

int main()
{
        double r,a,re,da,nr,rad;
        cin>>r>>a;
        re=r;
        nr=floor((double)180/a);
        da=f(r,2*a);
        re+=((nr-1)*da);
        double mod=360-floor(360/(2*a))*(2*a);
        //cout<<mod<<'\n';
        if(mod) re+=f(r,mod);
        cout<<fixed<<setprecision(6)<<re;
        return 0;
}

Algorithm right, but what in 8 test? Please, give me test;

Many times my code was reviewed and rewatched, but still WA7.
Could anybody help? What wrong with code? May be this algorithm gives not the optimal path?

int main()
{
    double r, a;
    std::cin >> r >> a;
    // first, walk to the edge of the island
    double s = r;
    // total angle of undiscovered edge
    const double a_left = 360.0 - 2.0*a;
    // hops count on 2*a "arc" line
    const int full_hops = (int)(a_left / (2.0*a));
    s += range(r, to_rad(2.0*a)) * full_hops;
    // remaining angle to hop
    double a_rem = a_left - full_hops * (2.0 * a);
    s += range(r, to_rad(a_rem));
    std::cout << std::fixed << std::setprecision(12) << s << std::endl;
}

Test is
10.000 29.999

Correst answer is
60.000583

What is wrong?
My answer is:
ans=((int)(360/(2*a)-1))*2*r*sin((a*pi)/180)+r

For accuracy of calculations use power-reduction formulas /* 2 * sin ^ 2 (a) = 1 - cos(2a) */ in the formula that calculates chords long.