编写C语言程序：输入一个正整数n，求S=1-1/2+2/3-3/5+5/8-8/13+???的前n项之和？

#include int main() { int n,i,a,b; double sum; scanf("%d",&n); for(i=0,a=b=1,sum=0;i<n;++i) { sum+=(double)a/b*(i%2?-1:1); b=a+b; a=b-a; } printf("%lf",sum); return 0; }