#include<iostream>
#include<cmath>
using namespace std;
int main()
{
    float a,b,c;
    float p,s;
    cin>>a>>b>>c;
    if(a+b>c&&a+c>b&&b+c>a)
    {
        p=(a+b+c)/2;
        s=sqrt(p*(p-a)*(p-b)*(p-c));
        cout<<"s="<<s;
    }
    else
    {
        cout<<"no";
    }
    return 0;
}