Голодная коза на веревке.

/* Thread 83603 */
 
#include <stdio.h>
#include <assert.h>
#include <math.h>
 
 
/********************************************************************/
#define PI 3.141592653589793238462
 
 
/********************************************************************/
double calc_koza_square( double N, double r );
 
 
/********************************************************************/
int main( void ) {
 
int N_int, r_int;
double S;
 
 
freopen( "input.txt", "r", stdin );
freopen( "output.txt", "w", stdout );
 
scanf( "%d %d", &N_int, &r_int );
assert( 1<=N_int && N_int<=100 );
assert( 1<=r_int && r_int<=100 );
 
S= calc_koza_square( (double)N_int, (double)r_int );
printf( "%.3f\n", S );
 
return 0;
 
} /* main() */
 
 
/********************************************************************/
double calc_koza_square( double N, double r ) {
 
double N_2, N2, teta, S;
 
 
N_2= 0.5*N;
N2= N/sqrt( 2.0 );
 
if ( r<=N_2 ) {
    return PI*r*r;
} else if ( r>=N2 ) {
    return N*N;
}
 
teta= 2*acos( N_2/r );
S= PI*r*r-4*(0.5*r*r*(teta-sin(teta)));
 
return S;
 
} /* calc_koza_square() */