#include <iostream>
#include <algorithm>

using namespace std;

int main(){
    int F, R, p, dias=0;
    cin >> F >> R;
    int fita[F+1];
    fill(fita, fita+F, 0);
    for(int i=0;i<R;i++){
        cin >> p;
        fita[p+1]=1;
    }
    int j=1;
    while(count(fita+1, fita+F, 0)!=0){
        dias++;
        for(int i=1;i<=F;i++){
            if(fita[i]=j){
                fita[i-1]=j+1;
                fita[i+1]=j+1;
            }
        }
        j++;
    }
    cout << dias << endl;
    return 0;
}