666新手被硬控了

#include <iostream>
#include <cstdio>

using namespace std;
int main(){
double F=1;
int n;
int i=1,j=1;
double jiecheng=1;
cin>>n;
while(j<=n){
jiecheng=1;
while(i<=j){jiecheng*=i;i++;}
i=1;
if(j%2!=0){F-=(1/jiecheng);}else{F+=(1/jiecheng);}
j++;
}
printf("%.8f",F);

return 0;

}