2-15求前n项和

#include<stdio.h>
int main(void){
int n,i,counter;
double sum;
i=1;
sum=0;
scanf("%d",&n);
for(counter=1;counter<=n;counter++){
if(counter%2==0) sum=sum-1.0/i;
else sum=sum+1.0/i;
i+=3;
}
printf("sum=%lf",sum);
return 0;
}