#include <stdio.h>
#include <math.h>
#define N (6)
namespace shukudai{
struct complex {
double re;
double im;
complex(double a,double b){
re=a;
im=b;
}
};
}
double Distance(struct shukudai::complex c0, struct shukudai::complex c1) {
return sqrt((c0.re-c1.re)*(c0.re-c1.re)+(c0.im-c1.im)*(c0.im-c1.im));
}
int main(void) {
int i;
double PI = 3.1415926535897931;
double sum = 0;
for (i=0 ; i<N ; i++){
int j = (i+1) % N;
sum += Distance(
shukudai::complex(cos(2*PI*i/N),sin(2*PI*i/N)),
shukudai::complex(cos(2*PI*j/N),sin(2*PI*j/N))
);
}
printf("%f\n",sum);
return 0;
}