c++判断并求三角形面积

#include<bits/stdc++.h>
using namespace std;
int main()
{
 double a,b,c,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<<"can't"<<endl;
 return 0;
}