void readfile()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);cout.tie(0);
    if (fopen(Task".inp","r"))
    {
        freopen(Task".inp","r",stdin);
        freopen(Task".out","w",stdout);
    }
    cin >> k;
    cin >> n;
    for(int i=1; i<=n; i++) cin >> a[i];
}

void proc()
{
    double prev_time = 0;
    for(int i=1; i<=n; i++){
        //first i person consume prev_time
        //if i+1 can hear i lesser than prev_time
        if (a[i+1] - prev_time < a[i]+k+1e-9) a[i+1] = max(a[i]+k,a[i+1]-prev_time);
        //else i+1 must run to the left prev_time distances
        else a[i+1]-=prev_time;
        prev_time += max(0.0,(a[i+1]-a[i]-k)/2);
        a[i+1] -= max(0.0,(a[i+1]-a[i]-k)/2);
    }
    cout << setprecision(6) << fixed << prev_time;
}