AC代码

#include <bits/stdc++.h>
using namespace std;
int calculate(int n) {
long long sum = 1;
for (int i = n; i >= 1; i--) {
sum *= i;
}
return sum;
}
int main() {
int n;
cin >> n;
double f = 1;
if (n == 1) {
cout << "0.00000000";
return 0;
}
for (int i = 1; i <= n; i++) {
if (i % 2 == 0) {
f = f + (1.0 / calculate(i));
}
else {
f = f - (1.0 / calculate(i));
}
}
cout << fixed << setprecision(8) << f;
}