#include <bits/stdc++.h>
#include "gap.h"
using namespace std;
typedef long long ll;
//#define DEBUG
#ifdef DEBUG
int n = 5;
ll arr[105] = {2, 3, 6, 8, 12};
void MinMax(ll x, ll y, ll *mn, ll *mx) {
for (int i = 0; i < n; ++i) if (arr[i] >= x)
{ (*mn) = arr[i]; break; }
for (int i = n - 1; i >= 0; --i) if (arr[i] <= y)
{ (*mx) = arr[i]; break; }
if ((*mx) < (*mn)) (*mx) = (*mn) = -1;
}
#endif
ll findGap(int _t, int _n) {
ll n = _n;
ll mn = 1, mx = (ll)1e18;
MinMax(mn, mx, &mn, &mx);
if (_t == 2) {
ll d = (mx - mn) / (n - 1ll) + (ll)((mx - mn) % (n - 1ll) != 0ll), pre = mn;
ll pstep = mn, step = mn + d - 1, gap = 0;
while (1) {
ll mntmp = -1, mxtmp = -1;
if (pstep < min(mx - 1ll, step)) MinMax(pstep + 1ll, min(mx - 1ll, step), &mntmp, &mxtmp);
if (step >= mx - 1ll) {
gap = max(gap, mntmp - pre);
if (mxtmp != -1ll) gap = max(gap, mx - mxtmp);
else gap = max(gap, mx - pre);
break;
} else if (mntmp != -1ll) gap = max(gap, mntmp - pre);
pre = max(pre, mxtmp), pstep = step, step += d;
}
return gap;
} else {
vector<ll> a;
a.resize(_n);
a[0] = mn, a[n - 1] = mx;
int lo = 0, hi = _n - 1;
while (hi - lo > 1) {
MinMax(a[lo] + 1ll, a[hi] - 1ll, &mn, &mx);
++lo, --hi;
a[lo] = mn, a[hi] = mx;
}
ll gap = 0; int pos = 0;
for (int i = 0; i < _n - 1; ++i) {
if (gap < a[i + 1] - a[i])
gap = a[i + 1] - a[i], pos = i;
}
return gap;
}
return 0ll;
}
#ifdef DEBUG
int main() {
freopen("input.txt", "rt", stdin);
int t;
scanf("%d%d", &t, &n);
for (int i = 0; i < n; ++i) scanf("%lld", &arr[i]);
printf("%lld", findGap(t, n));
return 0;
}
#endif
