#include <bits/stdc++.h>
using namespace std;
int main(){
  cout << fixed << setprecision(20);
  int N, M;
  cin >> N >> M;
  vector<int> B(M);
  for (int i = 0; i < M; i++){
    cin >> B[i];
    B[i]--;
  }
  double tv = N, fv = 0;
  for (int i = 0; i < 100; i++){
    double mid = (tv + fv) / 2;
    double sum = 0;
    for (int j = 0; j < M; j++){
      sum += min(mid + 1, (double) N - B[j]);
    }
    sum /= M;
    if (sum < mid){
      tv = mid;
    } else {
      fv = mid;
    }
  }
  cout << min((double) N - 1, B[0] + tv) << endl;
}