《一团代码》

15行的代码！！

using namespace std;
int main() {
    int n;
    cin >> n;
    double f = 1.0,factorial = 1.0;
    for (int i = 1; i <= n; ++i){
        factorial *= i;       
        double term = 1.0 / factorial;
        if (i % 2 == 1)f -= term;           
        else f += term;  }     
    cout << fixed << setprecision(8) << f ;
    return 0;}