#include <iostream>
using namespace std;
int N;
int K;
int ar[110000];
int s = 1,e,ans,mid;
int main() {
cin >> N >> K;
ans = N;
e = N;
for(int i = 0; i < K; i++) {
cin >>ar[i];
}
while(s<=e) {
mid = (s+e)/2;
bool check = true;
if(0 < ar[0]-mid || ar[K-1]+mid <N) check = false;
for(int i = 1; i <K; i++) {
if(ar[i]-mid > ar[i-1]+mid) check = false;
}
if(check) {
if(ans > mid) {
ans = mid;
}
e = mid -1;
}
else {
s = mid + 1;
}
}
cout<<ans;
}