求π的近似值

#include<bits/stdc++.h>
double funpi(double n)
{
    double w = 0,item = 1,flag=1,denominator=1;

    while(fabs(item)>=n){
        item = flag*1.0/denominator;
        w = w+item;
        flag = -flag;
        denominator = denominator + 2;
    }
    return 4*w;
}

int main()
{
    double pi,e;
    scanf("%lf",&e);
    pi = funpi(e);
    printf("pi = %lf",pi);
    return 0;
}