#include <stdio.h>
#include <math.h>
double mianji(double a, double b, double c); //定义计算面积的函数
int main()
{
double a, b, c;
scanf("%lf %lf %lf", &a, &b, &c);
printf ("%lf", mianji(a, b, c));
return 0;
}
double mianji(double a, double b, double c)
{
if ((a + b < c) || (a + c < b) || (b + c < a)) //是否满足三角形成立的条件判断
printf("error");
else
{
double p = (a + b + c) / 2;
double s = sqrt(p * (p - a) * (p - b) * (p - c));
return s;
}
}