计算交错序列前n项和

#include <stdio.h>
#include <math.h>
int main()
{
    int i,n;
    float res;
    scanf("%d", &n);
    for(i = 1;i <= n;i++){
        res += pow(-1,i-1)*i/(2*i-1);
    }
    printf("%.3f",res);
    return 0;
}

不知道对不对，供参考：

public static void main(String[] args) {
        // TODO Auto-generated method stub
        
        Scanner sc = new Scanner(System.in);
        int n =  sc.nextInt();
        double sum = 0;
        double num ;
        DecimalFormat df = new DecimalFormat(".000");
        
        for(int i=1;i<=n;i++) {
            
            if(i%2!=0) {
                num = (double)i/(2*i-1);
            }else {
                num = -(double)i/(2*i-1);
            }
            sum+=num;
        }
        sum = Double.valueOf(df.format(sum));
        System.out.println("sum="+sum);
    }