#include <bits/stdc++.h>
using namespace std;

#define REP(i,n) for(ll i=0;i<(ll)n;i++)
#define dump(x)  cerr << "Line " << __LINE__ << ": " <<  #x << " = " << (x) << "\n";
#define spa << " " <<
#define fi first
#define se second
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()

using ld = long double;
using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<ld, ld>;

template<typename T> using V = vector<T>;
template<typename T> using P = pair<T, T>;
template<typename T> vector<T> make_vec(size_t n, T a) { return vector<T>(n, a); }
template<typename... Ts> auto make_vec(size_t n, Ts... ts) { return vector<decltype(make_vec(ts...))>(n, make_vec(ts...)); }
template<class S, class T> ostream& operator << (ostream& os, const pair<S, T> v){os << "(" << v.first << ", " << v.second << ")"; return os;}
template<typename T> ostream& operator<<(ostream &os, const vector<T> &v) { for (auto &e : v) os << e << ' '; return os; }
template<class T> ostream& operator<<(ostream& os, const vector<vector<T>> &v){ for(auto &e : v){os << e << "\n";} return os;}
struct fast_ios { fast_ios(){ cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;

template <class T> void UNIQUE(vector<T> &x) {sort(ALL(x));x.erase(unique(ALL(x)), x.end());}
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
void fail() { cout << -1 << '\n'; exit(0); }
inline int popcount(const int x) { return __builtin_popcount(x); }
inline int popcount(const ll x) { return __builtin_popcountll(x); }
template<typename T> void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cerr<<v[i][0];for(ll j=1;j<w;j++)cerr spa v[i][j];cerr<<"\n";}};
template<typename T> void debug(vector<T>&v,ll n){if(n!=0)cerr<<v[0];
for(ll i=1;i<n;i++)cerr spa v[i];
cerr<<"\n";};

const ll INF = (1ll<<62);
// const ld EPS   = 1e-10;
// const ld PI    = acos(-1.0);
const ll mod = (int)1e9 + 7;
//const ll mod = 998244353;

int main(){

    ll N, M;
    cin >> N >> M;
    V<ll> X(M), Y(M);
    REP(i, M) cin >> X[i] >> Y[i];

    set<ll> st;
    st.insert(N);

    V<ll> ids(M, 0);
    iota(ALL(ids), 0);
    sort(ALL(ids), [&](ll x, ll y){
        return X[x] < X[y];
    });

    REP(i, M){
        ll j = i;
        while(j < M and X[ids[i]] == X[ids[j]]){
            j++;
        }
        ll len = j - i;
        V<ll> check(len, 0);
        V<ll> vals;
        for(ll k=i;k<j;k++){
            vals.push_back(Y[ids[k]]);
        }
        REP(k, len){
            if(st.find(vals[k]-1) != st.end() or st.find(vals[k]+1) != st.end()){
                check[k] = 1;
            }
        }
        REP(k, len){
            if(check[k]){
                st.insert(vals[k]);
            }else{
                auto itr = st.find(vals[k]);
                if(itr != st.end()) st.erase(itr);
            }
        }

        i = j - 1;

        // for(auto x: st) cerr << x << " ";
        // cerr << endl;
    }

    cout << st.size() << endl;

    return 0;
}
// Jai Shree Ram  
  
#include<bits/stdc++.h>
using namespace std;

#define rep(i,a,n)     for(int i=a;i<n;i++)
#define ll             long long
#define int            long long
#define pb             push_back
#define all(v)         v.begin(),v.end()
#define endl           "\n"
#define x              first
#define y              second
#define gcd(a,b)       __gcd(a,b)
#define mem1(a)        memset(a,-1,sizeof(a))
#define mem0(a)        memset(a,0,sizeof(a))
#define sz(a)          (int)a.size()
#define pii            pair<int,int>
#define hell           1000000007
#define elasped_time   1.0 * clock() / CLOCKS_PER_SEC



template<typename T1,typename T2>istream& operator>>(istream& in,pair<T1,T2> &a){in>>a.x>>a.y;return in;}
template<typename T1,typename T2>ostream& operator<<(ostream& out,pair<T1,T2> a){out<<a.x<<" "<<a.y;return out;}
template<typename T,typename T1>T maxs(T &a,T1 b){if(b>a)a=b;return a;}
template<typename T,typename T1>T mins(T &a,T1 b){if(b<a)a=b;return a;}


int solve(){
 		int n,m; cin >> n >> m;
 		map<int,vector<int>>col;
 		set<pii>st;
 		map<int,set<int>>mp;
 		mp[n].insert(0);
 		col[n].push_back(0);
 		for(int i = 1; i <= m; i++){
 			int x,y; cin >> x >> y;
 			st.insert({x,y});
 			col[y].push_back({x});
 			mp[y].insert(x);
 		}
 		for(auto &i:col){
 			sort(all(i.y));
 		}
 		set<pii>reached;
 		set<int>ans;
 		reached.insert({0,n});
 		for(auto [x,y]:reached){
 			if(st.count({x+1,y-1})){
 				reached.insert({x+1,y-1});
 				st.erase({x+1,y-1});
 				mp[y-1].erase({x+1});
 			}
 			if(st.count({x+1,y+1})){
 				reached.insert({x+1,y+1});
 				st.erase({x+1,y+1});
 				mp[y+1].erase(x+1);
 			}
			int j = upper_bound(all(col[y]),x) - col[y].begin();
			int row;
			if(col[y].back() == x) row = 2*n;
			else row = col[y][j];
			vector<pii>remove;
			auto itr = mp[y+1].upper_bound(x);
			while(itr != mp[y+1].end() and *itr <= row){
				reached.insert({*itr,y+1});
				remove.push_back({*itr,y+1});
				itr++;
			}
			itr = mp[y-1].upper_bound(x);
			while(itr != mp[y-1].end() and *itr <= row){
				reached.insert({*itr,y-1});
				remove.push_back({*itr,y-1});
				itr++;
			}
			for(auto [i,j]:remove){
				st.erase({i,j});
				mp[j].erase(i);
			}
			if(col[y].back() == x)ans.insert(y);
 			
 			
 		}
 		cout << ans.size() << endl;
 return 0;
}
signed main(){
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
    #ifdef SIEVE
    sieve();
    #endif
    #ifdef NCR
    init();
    #endif
    int t=1;//cin>>t;
    while(t--){
        solve();
    }
    return 0;
}
 