题解

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