error:'main' must return 'int'

#include<iostream.h>
using namespace std;

//类的定义;
class Circle
{
      public:
             float radius;//圆的半径
             float area();//圆的面积
             float perimeter();//圆的周长
};
//类的实现
float Circle::area()
{
      return radius*radius*3.14159;
}
float Circle::perimeter()
{
      return 2*3.14159*radius;
}
//类的使用对象
void main()
{
     Circle c;
     float areaOfc,perimeterOfc;
     
     cout<<"请输入圆的半径：";
     cin>>c.radius;
     if(c.radius<0)
         cout<<"半径不能为负数!\n";
     else
     {
         areaOfc=c.area();
         perimeterOfc=c.perimeter();
         cout<<"半径为"<<c.radius<<"的圆的面积为："<<areaOfc;
         cout<<"半径为"<<c.radius<<"的圆的面积为："<<perimeterOfc;
     }
}

编译后为什么 错误 1，'main'函数必须返回’int' ?