题解

#include<iostream>
using namespace std;
int main(){
int n; cin >> n;
double ans = 0;
for(int i = 1;i <= n;i++){
if(i % 2 == 1) ans += 1.0 / (double)i;
else ans -= 1.0 / (double)i;
}
printf("%.3lf",ans);
return 0;
}