题解

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