这样子?
> 
> #include <iostream>
> using namespace std;
> 
> int main()
> {
> 	double sumn = 0 , month;
> 	int i = 1;
> 	while (i <= 13)
> 	{
> 		cin >> month;
> 		sumn = sumn + month;
> 		++i;
> 	}
> 	sumn = sumn / i;
> 	printf("$%.2f\n" , sumn);
> 	return 0;
> }