Area of circle2

Source Code:
#include<stdio.h>
#include<math.h>
double area(double radius)
{
double pi = acos(-1); //remember that cos(pi) is -1 so acos(-1) is pi.
//complete this function to return the area.
     return pi*radius*radius;
}
int main()
{
double r;;
scanf("%lf", &r);
double a = area(r);
printf("%.6lf", a);
return 0;
}