自动推断返还类函数

#include<iostream>
using namespace std;

const double Pi =3.14159265;

auto Area(double radius)                                 编译器一直提示这里出错，是什么原因啊？怎么消除这个错误啊？感谢各位大神了！
{
    return Pi * radius * radius;
 }
 
 int main()
 {
     cout<<"Enter radius:";
     double radius=0;
     cin>>radius;
     
     //Call function"Area"
     cout<<"Area is:"<<Area(radius)<<endl;
     
     return 0;