c语言分子编写程序,输入一个正整数n,计算1-2/3+3/5-4/7+5/9-6/11+.的前n项之和.

/* 1-2/3+3/5-4/7+5/9-6/11+.的前n项之和*/
int main(int argc,char *argv[])
{
int num_n = 1;
int i = 0;
double result = 0.0;
printf("please input a Integer:");
//scanf("%d",&num_n);
num_n = 3;
for(i=0; i