Area of Circle in C

This program to find the area of the circle
Area of a circle is pi * r * r
where r is a radius of a circle.
Note:use Math.PI constant to get value of pi
This programs gets the input as radius of the circle from the user and do the calculation and display the result

Radius of a circle is 5
Then Result=3.14 * 5 * 5

Output:Area of a circle is 78.53981633974483
#include <stdio.h> 
#define PI 3.14159
int main()
float sum=0;
float r;
printf("Enter the radius of the circle\n");
sum = PI*r*r;
printf("The area of a circle is %f\n",sum);
return 0;

Enter radius of the circle 8
the area of a circle is 201.06192982974676