https://img-mid.csdnimg.cn/release/static/image/mid/ask/236603388666184.jpeg
#include<stdio.h>
#include<math.h>
double s(int n){
double sum = 0;
for(int i = 0;i < n;i++){
sum += (n + 1.0 * i);
}
return sum;
}
int main(void){
int n,sign = 1;
double y = 0;
scanf("%d",&n);
for(int i = 1;i <= n;i++){
double fenmu = s(i);
double fenzi = pow(i,2) * sign;
y += (fenzi / fenmu);
sign *= -1;
}
printf("%lf",y);
return 0;
}