#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <ctype.h>
#pragma warning(disable:4996)
typedef long long s64, LL;
typedef unsigned long long u64, ULL;
typedef const void *cvp;
#define MAX(a,b) (((a)>(b))?(a):(b))
int cmpi32(const void *p, const void *q){
    int a = *((int *)p), b = *((int *)q);
    if(a > b)return 1;
    else if(a < b)return -1;
    else return 0;
}
int arr[1919810] = { 0 };
int main(){
#ifdef _DEBUG
    FILE *fp = freopen("../../../input.txt", "r", stdin);
    //FILE *fp2 = freopen("../../../output.txt", "w", stdout);
#endif // _DEBUG
    int n, m, i, t;
    scanf("%d%d", &n, &m);
    for(i = 0; i < m; i++){
        scanf("%d", arr + i);
    }
    qsort(arr, m, sizeof(int), cmpi32);
    t = MAX(arr[0] - 1, n - arr[m - 1]);
    for(i = 1; i < m; i++){
        t = MAX(t, (arr[i] - arr[i - 1]) / 2);
    }
    printf("%d\n", t + 1);
#ifdef _DEBUG
    fp = freopen("CON", "r", stdin);
    //fp2 = freopen("CON", "w", stdout);
    system("pause");
#endif // _DEBUG
    return 0;
}