#include <iostream>
#include <iomanip>
using namespace std;
typedef long long ll;

int main()
{
    int n, m;
    cin >> n >> m;
    int b[100005];
    for(int i = 0; i < m; i++) cin >> b[i];
    double ans = n - 1;
    ll s = 0;
    for(int i = m - 1; i > 0; i--){
        s += n - b[i];
        ans = min(ans, b[0] - 1 + m * 1.0 / (m - i) + s * 1.0 / (m - i));
    }
    cout << fixed << setprecision(15) << ans << endl;
}