看我！！！

ac	wa
9999999999999999999999999999999999999999999999999999999999999999999	0

#include<iostream>
#include<cstdio>
using namespace std;
int main(){
int n;
double f=1;
cin >> n;
for(int i=1;i<=n;i++){
int x=1;
for(int j=1;j<=i;j++){
x*=j;
}
if(i%21)
f-=1.0/x;
else if(i%20)
f+=1.0/x;

}
printf("%.8lf",f);

}