#include <bits/stdc++.h>
#ifndef SKY
#include "gap.h"
#endif // SKY
using namespace std;
#define ll long long
#define pb push_back
#define N 200010
#define ii pair<int,int>
#define fs first
#define sc second
#define ld double
int n;
#ifdef SKY
ll a[N],am=-1,mn,mx;
void MinMax(ll s,ll t, ll *u, ll *v)
{
if(s>t)
{
cout<<"NGU VC";
exit(0);
}
if(s>a[n]||a[lower_bound(a+1,a+1+n,s)-a]>t)
{
mn=-1;
mx=-1;
return;
}
mn=a[lower_bound(a+1,a+1+n,s)-a];
if(t>=a[n])
{
mx=a[n];
return;
}
mx=a[prev(upper_bound(a+1,a+1+n,t))-a];
}
#endif
long long findGap(int T, int cc)
{
long long a[11111111];
memset(a, 0, sizeof a);
for (int i = 0; i < n; i++) a[i] = i ^ (i >> 1);
for (int i = 0; i < n; i++) a[i] = a[i] ^ (a[i] & (1 << i));
n=cc;
ll L=0,R=1e18;
vector<ll>s;
int cnt=0;
while(L<=R)
{
cnt++;
if(cnt*2>(n+1))
break;
ll u,v;
MinMax(L,R,&u,&v);
#ifdef SKY
u=mn;v=mx;
#endif // SKY
//cout<<L<<" "<<R<<" "<<u<<" "<<v<<endl;
if(u==-1)
break;
s.pb(u);
s.pb(v);
L=u+1;
R=v-1;
}
sort(s.begin(),s.end());
s.erase(unique(s.begin(),s.end()),s.end());
ll res=0;
for(int i=0;i<s.size()-1;i++)
res=max(res,s[i+1]-s[i]);
return res;
}
#ifdef SKY
int main()
{
freopen("A.inp","r",stdin);
freopen("A.out","w",stdout);
int n,T;
cin>>T>>n;
for(int i=1;i<=n;i++)
cin>>a[i];
cout<<findGap(T,n)<<endl;
return 0;
}
#endif // SKY
