//三条辅助线
//扇形面积减两块
//可以想一下辛普森积分
#include <bits/stdc++.h>
#define pi 3.14159265
using namespace std;
int main()
{
double r,a,b;
scanf("%lf%lf%lf",&r,&a,&b);
double x1,y1,x2,y2,l,p,s,l2,l3,n,S;
x1=a;
y1=sqrt(r*r-a*a);
x2=sqrt(r*r-b*b);
y2=b;
l=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
p=(r+r+l)/2.00;
s=sqrt(p*(p-r)*(p-r)*(p-l));
l2=2*s/r;
l3=sqrt(r*r-l2*l2);
n=acos(l3/r);
S=n*pi*r*r/(2*pi)-0.5*(y1-b)*a-0.5*(x2-a)*b;
printf("%.2lf\n",S);
return 0;
}
