#include "gap.h"
#include <bits/stdc++.h>
using namespace std;
long long a[100005];
//void MinMax(long long s, long long t, long long *mn, long long *mx);
long long scamBucket[100005];
long long findGap(int T, int N){
if(T == 1){
MinMax(0, 1LL<<60, &a[0], &a[N-1]);
for(int i = 1; i < (N+1)/2; i ++){
MinMax(a[i-1]+1, a[N-i]-1, &a[i], &a[N-i-1]);
}
long long ans = 0;
for(int i = 0; i < N-1; i ++){
//printf("%d %d\n", i, a[i]);
ans = max(ans, a[i+1]-a[i]);
}
}else{
MinMax(0, 1LL<<60, &a[0], &a[N-1]);
int scamFactor = N-2;
vector<long long> scam;
scam.push_back(a[0]);
for(int i = 0; i < scamFactor; i ++){
scamBucket[i] = a[0] + (a[N-1]-a[0])/scamFactor*i;
}
scamBucket[scamFactor] = a[N-1];
for(int i = 0; i < scamFactor; i ++){
long long x, y;
MinMax(scamBucket[i]+1, scamBucket[i+1]-(i!=scamFactor-1), &x, &y);
if(x != -1){
scam.push_back(x);
scam.push_back(y);
}
}
scam.push_back(a[N-1]);
long long ans = 0;
for(int i = 0; i < (int)scam.size()-1; i ++){
//printf("%d %d\n", i, a[i]);
ans = max(ans, scam[i+1]-scam[i]);
}
return ans;
}
}
/*
static void my_assert(int k){ if (!k) exit(1); }
static int subtask_num, N;
static long long A[100001];
static long long call_count;
void MinMax(long long s, long long t, long long *mn, long long *mx)
{
int lo = 1, hi = N, left = N+1, right = 0;
my_assert(s <= t && mn != NULL && mx != NULL);
while (lo <= hi){
int mid = (lo+hi)>>1;
if (A[mid] >= s) hi = mid - 1, left = mid;
else lo = mid + 1;
}
lo = 1, hi = N;
while (lo <= hi){
int mid = (lo+hi)>>1;
if (A[mid] <= t) lo = mid + 1, right = mid;
else hi = mid - 1;
}
if (left > right) *mn = *mx = -1;
else{
*mn = A[left];
*mx = A[right];
}
if (subtask_num == 1) call_count++;
else if (subtask_num == 2) call_count += right-left+2;
}
int main()
{
FILE *in = stdin, *out = stdout;
my_assert(2 == fscanf(in, "%d%d", &subtask_num, &N));
my_assert(1 <= subtask_num && subtask_num <= 2);
my_assert(2 <= N && N <= 100000);
for (int i=1;i<=N;i++) my_assert(1 == fscanf(in, "%lld", A+i));
for (int i=1;i<N;i++) my_assert(A[i] < A[i+1]);
fprintf(out, "%lld\n", findGap(subtask_num, N));
fprintf(out, "%lld\n", call_count);
}
*/
