include<stdio.h>

include<math.h>

int main() {

int a,b,c;
double p,s,t;
int A,B,C;
scanf("%d %d %d",&a,&b,&c);
A=a+b;
B=a+c;
C=b+c;
if(A>c&&B>b&&C>a)
{
    p=(a+b+c)/2;
    t=p*(p-a)*(p-b)*(p-c);
    s=sart(t);
    printf("%.2lf",s);
}
else
    printf("No")
return 0;

}