Formula for calculating pi pai: Pai = 4* (1-1/3+1/5-1/7 ...)

There are many formulas for calculating pi pai in history, in which Gregory and Leibniz found the following formula:

Pai = 4* (1-1/3+1/5-1/7 ...)

The formula is simple and graceful, but in a bad way, it converges too slowly.
If we rounded to keep its two decimal digits, then:

Cumulative 1 items and are: 4.00
Cumulative 2 items and are: 2.67
Cumulative 3 items and are: 3.47
。。。

Please write out what it has accumulated 100 items and how much (rounded to two digits after the decimal).

Source:

#include <stdio.h>
//pai = 4* (1-1/3+1/5-1/7 ...)
Double while ()
{
	double sum=0;
	int flag=1;//odd Item bit positive, even term is negative
	double temp=1;
	int i=1;
	while (i<=100)
	{
		temp=1.0/(2*i-1) * (flag);
		i++;
		Sum=sum+temp;
		Flag=-flag;
	}
	return sum; 
}
int main ()
{
	double pai=0.0;
	Pai=4*while ();
	printf ("%.8lf\n", pai);
	return 0;
}

Output: 3.13159290, so rounded to 3.13