code/class/e_de_jin_se_zhi.cpp

#include<iostream>
#include<iomanip>

using namespace std;

int main(){
    double e = 1.0;
    double factorial = 1.0;

    for (int i = 1; i <= 20; i++)
    {
        factorial *= i;
        e += 1.0 / factorial;
    }

    cout << fixed << setprecision(3) << e << endl;
    
    return 0;
}