#include<iostream>
#include<iomanip>
using namespace std;
int N,M;
int B[1<<17];
main()
{
	cin>>N>>M;
	for(int i=0;i<M;i++)cin>>B[i];
	double L=0,R=2e9;
	for(int c=0;c<100;c++)
	{
		double mid=(L+R)/2;
		double now=0;
		for(int i=0;i<M;i++)
		{
			double x=N-B[i];
			if(x<mid)now+=x;
			else now+=mid;
		}
		now/=M;
		now+=1;
		if(mid<now)L=mid;
		else R=mid;
	}
	double ans=L+B[0]-1;
	if(ans>N-1)ans=N-1;
	cout<<fixed<<setprecision(16)<<ans<<endl;
}