#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define str string
#define pb push_back
#define pf push_front
#define in insert
#define all(v) v.begin(),v.end()
#define fastIO ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
const int sz=1000000,INF=1000000000;
using namespace std;
void solve()
{
ll n,m,num,minNum,maxNum,ans=-INF;
cin>>n>>m;
vector<ll>nums;
for(int i=0;i<m;i++)
{
cin>>num;
nums.pb(num);
}
for(int i=0;i<nums.size();i++)
{
minNum=INF,maxNum=-INF;
for(int j=i;j<nums.size();j++)
{
minNum=min(minNum,nums[j]),maxNum=max(maxNum,nums[j]),ans=max(ans,maxNum-minNum-(j-i+1));
}
}
cout<<ans;
}
int main()
{
fastIO;
ll t=1;
//cin>>t;
while(t--)
{
solve();
}
}
