# include <stdio.h>
# define PI 3.141592654					// Defines value for PI
# define CalcArea(x) (x * x * PI)		// Defines function to calculate area


void main(void){						// Main function takes on input, and gives no output, hence void

	double Radius;						// Radius of circle
	double Area;						// Save calculated Area

	printf("Enter radius please: ");	// User-friendly, ask user for input

	scanf("%lf",&Radius);				// Wait for user to enter value

	Area = CalcArea(Radius);			// Calculate the area
	// Area = Radius * Radius * PI;		// Compiler rewrites previous line to be same as this one

	printf("Area is %lf\n",Area);		// Output area

}