calculo area de circulo

#include<stdio.h>
#include<stdlib.h>
double pi(int n);
int main(void)
{
double area,r;
int n;
printf("digite n\n");
scanf("%d",&n);
printf("digite raio\n");
scanf("%lf",&r);
printf("calculo area\n");
area=pi(n)*r*r;
printf("area circulo=%f\n",area);
system("pause");
return 0;
}
double pi(int n)
{
int i;
double s=0.0;
for(i=0;i<=n;i++)
{
if(i%2==0)
{
s=s+1.0/(2.0*i+1.0);
}
else
{
s=s-1.0/(2.0*i+1.0);
}
}
s=s*4.0;
return s;
}