/*
     Hell, 'til I reach Hell, I ain't scared
     Mama checkin' in my bedroom, I ain't there
                                                */
    #include<bits/stdc++.h>
    #include <ext/pb_ds/assoc_container.hpp>
    #include <ext/pb_ds/tree_policy.hpp>
    using namespace __gnu_pbds;
    using namespace std;

    #define Major  ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    #define TxtIO   freopen("input.txt","r",stdin); freopen("output.txt","w",stdout);
    #define read(a,n) for(int i = 0 ; i<n ; i++) cin>>a[i];
    #define write(a) for(auto x : a) cout<<x<<" ";
    #define int long long
    #define pb push_back
    #define all(a)  a.begin(),a.end()
    #define el "\n"

    typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;

    const int inf=4e18;

    int dx[]={1,0,-1,0,1,1,-1,-1};
    int dy[]={0,1,0,-1,1,-1,1,-1};

    const int mod=1e9+7;
    const int MAX =1e6+7;

   // ---------------------------Function----------------------------------//

    void TestCake(){
       int all,n;
       cin>>all>>n;
       vector<int>arr(n);
       read(arr,n);
       map<int,int>mp;
       for(auto x : arr)mp[x]++;
       int ans=0;
          map<int,int>tmp=mp;
          int cnt=0;
           for(int i=2;i<all;i++){
              if(!tmp[i]&&!tmp[(i-1)]&&!tmp[i+1]){
                cnt++;
                tmp[i]=1;
              }
           }
         if(!tmp[1]&&!tmp[all]&&!tmp[2]){
            tmp[1]++;
            cnt++;
         }
         if(!tmp[all]&&!tmp[all-1]&&!tmp[1]){
            cnt++;
         }
       cout<<cnt;
    }
    //------------------------------Main---------------------------//
    signed main(){
        Major
        int T = 1;
//        cin >> T;
        while(T--){
            TestCake();
        //       cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
        }
        return 0;
    }