[C language] PTA 7-52 finding the sum of the first n terms of a simple interleaved sequence

This question requires the preparation of procedures , Calculation sequence 1 - 1/4 + 1/7 - 1/10 + ... Before N Sum of items .

Input format :

Input gives a positive integer on a line N.

Output format :

In a row, press “sum = S” The format of the output part and the value of S, Accurate to three decimal places . The calculation results should not exceed the double precision range .

sample input :

10

No blank lines at the end

sample output :

sum = 0.819

No blank lines at the end

#include<stdio.h>
int main(){
	
	int N;
	double sum=0;
	double i;// The first i term 
	scanf("%d",&N);
	int sign=1;// Symbol 
	
	for(i=1;i<=N;i++){
		double a; 
		a=1/(3*i-2);
		sum=sum+sign*a;
		sign*=-1;
	}
	
	printf("sum = %.3f",sum);
	return 0;
}