#define _DEBUG
#include "bits/stdc++.h"
//#include <atcoder/all>
#define CHOOSE(a) CHOOSE2 a
#define CHOOSE2(a0,a1,a2,a3,a4,a5,x,...) x
#define debug_1(x1) cout<<#x1<<": "<<x1<<endl
#define debug_2(x1,x2) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<endl
#define debug_3(x1,x2,x3) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<endl
#define debug_4(x1,x2,x3,x4) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<", "#x4<<": "<<x4<<endl
#define debug_5(x1,x2,x3,x4,x5) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<", "#x4<<": "<<x4<<", "#x5<<": "<<x5<<endl
#define debug_6(x1,x2,x3,x4,x5,x6) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<", "#x4<<": "<<x4<<", "#x5<<": "<<x5<<", "#x6<<": "<<x6<<endl
#ifdef _DEBUG
#define debug(...) CHOOSE((__VA_ARGS__,debug_6,debug_5,debug_4,debug_3,debug_2,debug_1,~))(__VA_ARGS__)
#else
#define debug(...)
#endif
#define rep(index,num) for(int index=0;index<(int)num;index++)
#define rep1(index,num) for(int index=1;index<=(int)num;index++)
#define brep(index,num) for(int index=(int)num-1;index>=0;index--)
#define brep1(index,num) for(int index=(int)num;index>0;index--)
#define scan(argument) cin>>argument
#define prin(argument) cout<<argument<<endl
#define prind(argument) cout<<std::fixed<<setprecision(13)<<argument<<endl
#define kaigyo cout<<endl
#define eps 1e-7
#define mp(a1,a2) make_pair(a1,a2)
#define ALL(a) (a).begin(),(a).end()
#define rALL(a) (a).rbegin(),(a).rend()
#define puba(a) emplace_back(a)
#define pubamp(a,b) emplace_back(mp(a,b))
typedef long long ll;
typedef long double ld;
using namespace std;
//using namespace atcoder;
typedef pair<ll,ll> pll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<pint> vpint;
typedef vector<pll> vpll;
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
ll INFl=(ll)1e+18+1;
int INF=1e+9+1;
ll x[100001];
ll ruix[100001];
int main(){
	ll N,M;
	scan(N>>M);
	rep(i,M) scan(x[i]);
	if(M==1){
		prin(N-1);
		return 0;
	}
	brep(i,M) ruix[i]=ruix[i+1]+x[i];
	ld warpkitaimin=INFl;
	rep1(k,M-1){
		ld warpkitai=N-(ld)(ruix[k]-k)/(M-k);
		chmin(warpkitaimin,warpkitai);
	}
	prind(min((ld)(N-1),x[0]+warpkitaimin));
	return 0;
}