极为简易

#include <iostream>
long long f (long long n) {
    long long res = 1;
    for(long long i = 2;i <= n;i++) {
        res *= i;
    }
    return res;
}
int main () {
    long long n, sum = 0;
    std::cin >> n;
    for (long long i = 2;i <= n;i++) {
        sum += f(i);
    }std::cout << sum + 1;
}