Uri 2452 - Semente

/*                                      CODEMAN
                                     Arif khan Nihar
                                   Uri 2452 - Semente

                                 */
#include<bits/stdc++.h>
 using namespace std;

int main()
{
    int n,k = 0,ans = 0,l,i,j,x,y,res=0,p = 0;
    queue < int > pq;
    int a[100001];
        cin >> j >> y;
    for(i = 0;i < y;i++){
            cin >> k;
            pq.push(k);
        }
        a[0] = 1;
        for(i = 1;i <= j;i++){
            a[i] = 0;
        }
        l = i = y;
         k = 0;
    while(!pq.empty()){
           x = pq.front();
           res++;
           pq.pop();

            a[x] = 1;
            if(a[x-1] == 0 and x-1 > 0)
            pq.push(x-1),k++,a[x-1] = 1;
            if(a[x+1] == 0 and x+1<= j)
            pq.push(x+1),k++,a[x+1] = 1;

            if(res == l){
                l = k;
                k = 0;
                if(p != 0)
                ans++;
                res = 0;
                p++;
            }
    }
    cout << ans << endl;
    return 0;
}