#include <bits/stdc++.h>
using namespace std;
#define _MACRO(_1, _2, _3, NAME, ...) NAME
#define _repl(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define _rep(i,n) _repl(i,0,n)
#define rep(...) _MACRO(__VA_ARGS__, _repl, _rep)(__VA_ARGS__)
#define mp make_pair
#define pb push_back
#define all(x) begin(x),end(x)
#define uniq(x) sort(all(x)),(x).erase(unique(all(x)),end(x))
#define fi first
#define se second
#define dbg(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
void _dbg(string){cerr<<endl;}
template<class H,class... T> void _dbg(string s,H h,T... t){int l=s.find(',');cerr<<s.substr(0,l)<<" = "<<h<<", ";_dbg(s.substr(l+1),t...);}
template<class T,class U> ostream& operator<<(ostream &o, const pair<T,U> &p){o<<"("<<p.fi<<","<<p.se<<")";return o;}
template<class T> ostream& operator<<(ostream &o, const vector<T> &v){o<<"[";for(T t:v){o<<t<<",";}o<<"]";return o;}

int main(){
  int n,m;
  cin>>n>>m;
  vector<int> a(m);
  rep(i,m) cin>>a[i];

  int ans = max(a[0]-1, n - a.back());
  rep(i,m-1){
    ans = max(ans, (a[i+1] - a[i])/2);
  }
  cout << ans << endl;

  return 0;
}

