#include<iostream>
using namespace std;
#include <iomanip>
  int main()
{
	double a, b, c;
	cin >> a>>b>> c;
	double p = (a + b + c) / 2;
	double s2 = (p * (p - a) * (p - b) * (p - c));
	double s = sqrt(s2);
	cout<< fixed << setprecision(1) << s;
}
  

以上代码会提示我sqrt在此作用域中尚未声明，请问该如何修改呢？