后来想到可以用三目运算符简化代码

#include <stdio.h>
int main (){
        int n,i,temp;
        int sum=0;
        scanf("%d",&n);
        for(i=0;i<=n;++i){
                if(i%2==0){
                        temp=i*(-1);
                        
                }else{
                        temp=i;
                }
                sum=sum+temp;
        }
        printf("%d",sum);
        return 0;
}