#include "bits/stdc++.h"
using namespace std;
#define int long long
signed main() {
int n, m;
cin >> n >> m;
vector <int> a(m + 1);
for (int i = 1; i <= m; i ++) cin >> a[i];
vector <int> p(m + 1);
vector <int> s(m + 1);
// max ind min ind
int ans = -1;
p[1] = a[1] - 1;
s[m] = a[m] + 1;
for (int i = 2; i <= m; i ++) p[i] = max(a[i] - i, p[i - 1]);
for (int i = m - 1; i >= 1; i --) s[i] = min(s[i + 1], a[i] + m - i + 1);
for (int i = 1; i <= m; i ++) {
ans = max(ans, p[i] - s[i]);
}
cout << ans << endl;
return 0;
}
