题解 | #Balls#

Time: O(n)

Memory: O(1)

int n;
double s = 1.0 , p = 0.5;
//=================================
int main(){
    n=read();
    rep(i,1,n)
    	s = (s + 1.0) * p + s * (1.0 - p),p = 1.0 * s/(i+2.0);
    printf("%.7lf",s);
    return 0;
}