#include<iostream.h>
#include<math.h>
int main()
{
    double pi;
    double l, r,alpha,segm,treug,delta;
    cin>>l>>r;
    pi=3;
    l/=2;
    if(r<=l/2)
        cout<<pi*r*r;
    if(r>(l*sqrt(2)))
        cout<<l*l*4;
    else
    {
        alpha=2*acos(l/r);
        segm=pi*r/(2*alpha);
        treug=l*sqrt(((r*r)-(l*l)));
        delta=segm-treug;
        cout<<pi*r*r-4*delta;
    }
    return 0;
}

Sorry pi=3,145
Why when inpus is:
3 1
Output is:
-1.#IND
???
Compiler:
Visual C++ 6.0