#include <bits/stdc++.h>

#define ll long long

#define pb push_back
#define el '\n'
#define pi 3.1415926536
#define mod 1000000007

#include <sstream>

#define fast ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0)

#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;
template<typename T>
using pbds = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;


int const N = 2e5+5;

map<int, int>mp;
map<int, int> freqq;
map<pair<ll, ll>, ll> mpp;
vector<pair<ll,ll>>adj[N],adj2[N];
deque<pair<int,int>>pq2;
//priority_queue<pair<ll,ll>,vector<pair<ll,ll>>,greater<>>pq;

int dx[] = { 1, -1,0, 0 };
int dy[] = { 0,0, 1,-1  };
int chess_x[] = { -1,-1,1,1, 1, -1, 0, 0 ,0};
int chess_y[] = { 1, -1,1,-1, 0,0, 1,-1,0 };
ll a[N];

int main()
{
    fast;
   int s,n;
   cin>>s>>n;
   for(int i=0;i<n;i++)cin>>a[i];
   int ans=0;
   for(int i=0;i<n;i++){
       if(i!=n-1 && i){
           int dif=abs(a[i]-a[i-1])-2;
           if(dif>=3) {
               ans += dif / 2;
           }
       }
       else if(i==n-1){
           int dif=abs(a[i]-a[i-1])-2;
               ans += dif / 2;
           int dif2=s-a[i]+(abs(a[0]-1));

              // if (dif2 % 2 == 1)ans += dif2 / 2;
              ans += (dif2-1) / 2;
          // cerr<<dif<<" "<<dif2<< " ";
       }
   }
   cout<<ans;

    return  0;
}