/**
 *    author:  otera
 *    created: 06.03.2021 01:14:21 
**/
#include<bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

void solve() {
    int n, m; cin >> n >> m;
    vector<ll> b(m);
    vector<ll> sum(m + 1, 0);
    for(int i = 0; i < m; ++ i) {
        cin >> b[i];
    }
    for(int i = 0; i < m; ++ i) {
        sum[i + 1] = sum[i] + (b[m - 1] - b[m - 1 - i]);
    }
    ld ans = (ld)(n - 1);
    int ok = m - 1, ng = 0;
    auto check = [&](int x) -> bool {
        int right = b[m - 1] - b[x];
        ld left = ((ld)m + (ld)sum[m - x]) / (ld)(m - x);
        if((ld)left >= (ld)right) return true;
        else return false;
    };
    while(ok - ng > 1) {
        int mid = (ok + ng) / 2;
        if(check(mid)) ok = mid;
        else ng = mid;
    }
    ld q = ((ld)m + (ld)sum[m - ok]) / (ld)(m - ok);
    chmin(ans, (ld)(n - b[m - 1] + b[0] - 1) + q);
    cout << ans << endl;
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);
    // INT(t); rep(i, t)solve();
    solve();
    return 0;
}