#include <stdio.h>
#include <math.h>

//Calculates the area of a circle.
// r - radius of circle
// returns: area of circle with radius r
double area(double r);

int main(void) {

    double r;
    for (r = 1; r <= 5; r++)
        printf("Area of circle with radius %f is %f\n",
                r,
                area(r));
    return 0;
}

double area(double r) {
    return M_PI * r * r;
}