求简单交错序列前N项和

#include
int main()
{
int N,i;
double sum=0.0;
scanf("%d",&N);
int f = 1;
for(i=1;i<=N;i++)
{
sum = sum + f1.0/(3i-2);
f =-f;//符号正负变换
}
printf("sum = %.3lf",sum);
}

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