题解

#pragma message("A30835.This file is written by C language.")
extern "C"{
    #include<stdio.h>
    int n;long long sum;
    int main(){
        scanf("%d",&n);
        for (int i=1;i<=n;i++){
            if (i%2==0) sum+=-i;
            else sum+=i;
        }
        printf("%lld",sum);
        return 0;
    }
}