100%AC

#include<cstdio>
int f(double x){
    int product=1;
    for(int i=2;i<=x;i++){
        product*=i;
    }
    return product;
}
int main(){
    int F;double sum=0.;
    scanf("%d",&F);
    for(int i=2.;i<=F;i++){
        if(i%2==0){
            sum+=1./f(i);
        }else{
            sum-=1./f(i);
        }
    }
    printf("%.8lf",sum);
}