#include <atcoder/all>
#include <bitset>
#include <fstream>
#include <functional>
#include <iostream>
#include <iomanip>
#include <limits>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <stdio.h>
#include <stdlib.h>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;
using namespace atcoder;

//* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ *//

void input() {
    
    
}


void solve() {
    
    
    int N, M; cin >> N >> M;
    vector<long double> B(M);
    for(int i = 0; i < M; i++) {
        int b; cin >> b;
        B[M-i-1] = N-b;
    }
    
    long double ok = 0, ng = N;
    long double Es = 0;
    vector<long double> E(M);
    
    int roop = 100;
    while(roop--) {
        long double mn = (ok + ng) / 2;
        
        E.resize(M, 0);
        E[0] = min(B[0], mn + 1);
        Es = E[0];
        for(int i = 1; i < M; i++) {
            E[i] = min(B[i] - B[i-1] + E[i-1], mn + 1);
            Es += E[i];
        }
        
        Es /= (long double)M;
        Es > mn ? ok = mn : ng = mn;
    }
    
    long double ans = N - 1 - B[M-1] + E[M-1];
    
    cout << fixed << setprecision(6);
    cout << ans << endl;
    
    
    
    
}

//* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ *//

int main() {
    
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
    std::cin.tie(0);
    ios::sync_with_stdio(false);
    
    input();
    solve();
    
    return 0;
}