Calculate the value of PI in c++

The value of π can be determined by the series equation
π=4 ∗ ( 1-1/3 +1/5−1/7+1 / 9−1 / 11+1 / 13…)
Write a program that prompts the user to enter a positive odd number n and uses recursion to approximate the value of π using the given formula including term up through 1/n.

i have did the following.. rectify my mistake.. thanks in advance

#include <iostream>
using namespace std;

void main()
{
	int n;
	double f;
	double ans =0 ;
	cin >> n;

	for (int i=0; i<n ; i++)
	{
			f = 4 * (1/(2*i + 1));

			if ((i %2) == 1 )
			  f = -(f);
			else
				f=f;
	
			ans += f;
		}
	}
   cout << ans << endl;
}

kindly help me