{
	int@n,@m;
	VI@b(m);
	double s=0;
	vector<double> e(m);
	int x=n;
	{
		i=m-1;
		int d=x-b[i];
		double ea=d;
		e[i]=ea;
		s+=e[i];
		x=b[i];
	}
	rrep(i,m-1){
		int d=x-b[i];
		double ea=e[i+1]+d;
		double ew=(double)(m+s)/(m-i-1);
		e[i]=min(ea,ew);
		s+=e[i];
		x=b[i];
	}
	wt(b[0]-1+e[0]);
}