//wait darn
#include<bits/stdc++.h>
#include <stdio.h>
#include <stdlib.h>
#include "gap.h"
using namespace std;
typedef long long ll;
/*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;
long long findGap(int, int);
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);
}
*/
long long findGap(int T, int N)
{
if(T == 1) {
set<ll> s;
ll lo = 0, hi = 1000000000000000000;
ll fi, se;
for(int i = 0; 2*i < N; i++) {
MinMax(lo,hi,&fi,&se);
s.insert(fi);
s.insert(se);
lo = fi+1;
hi = se-1;
}
ll ans = 0;
set<ll>::iterator it2 = s.begin();
it2++;
for(set<ll>::iterator it = s.begin(); it2 != s.end();) {
ans = max(ans, (*it2)-(*it));
it++;
it2++;
}
return ans;
}
}
