#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using u64 = unsigned long long;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

int main() {
    int n, m;
    cin >> n >> m;
    double ans = n-1;
    vector<int> b(m);
    for (auto &&i : b) scanf("%d", &i);
    double k = m, c = 0;
    for (int i = m-1; i > 0; --i) {
        k += n-b[i];
        c += 1;
        ans = min(ans, b[0]-1 + k/c);
    }
    printf("%.10lf\n", ans);
    return 0;
}