CDAY

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

constexpr int N = 1e6 + 5;
constexpr int Inf = 1e9 + 7;

int n, t, f[N];
int a[N], b[N];
int l[N], r[N];
vector<int> startBy[N],
    endBy[N];

multiset<int> listl, listr; // "danh sách"

void Read()
{
    cin >> n >> t;

    for (int i = 1; i <= t; ++i)
        cin >> a[i] >> b[i];
}

struct SegmentTree {
    int n;
    int st[N * 4];

    SegmentTree() {
        fill_n(st, N * 4, -Inf);
    }
    void Update(int id, int l, int r, const int &x, const int &v) {
        if(l > x || r < x) return;
        if(l == x && r == x) {
            st[id] = v;
            return;
        }
        Update(id << 1, l, (l + r) / 2, x, v);
        Update(id << 1 | 1, (l + r) / 2 + 1, r, x, v);
        st[id] = max(st[id << 1], st[id << 1 | 1]);
    }
    void Update(int x, int v) {
        Update(1, 1, n, x, v);
    }

    int Get(int id, int l, int r, const int &a, const int &b) {
        if(l > b || r < a) return -Inf;
        if(l >= a && r <= b) return st[id];

        return max(Get(id << 1, l, (l + r) / 2, a, b), Get(id << 1 | 1, (l + r) / 2 + 1, r, a, b));
    }

    int Get(int l, int r) {
        return Get(1, 1, n, l, r);
    }
} g;

void Solve()
{
    for (int i = 1; i <= t; ++i)
    {
        startBy[a[i]].push_back(i);
        endBy[b[i]].push_back(i);
    }

    for (int i = 1; i <= n + 1; ++i)
    {
        for (auto proj : startBy[i])
            listr.insert(a[proj]); // Thêm phần tử

        r[i] = listr.empty() ? i : *listr.begin();
        l[i] = listl.empty() ? 0 : *listl.rbegin();

        for (auto proj : endBy[i])
        {
            listr.erase(listr.find(a[proj])); // Chỉ xóa một phần tử
            listl.insert(a[proj]);
        }
    }

    g.n = n + 2;
    g.Update(0 + 1, 0);
    //cerr << "ok\n";

    for(int i = 1; i <= n + 1; ++i) {
        //cout << i << ": " << l[i] << " " << r[i] << "\n";
        f[i] = g.Get(l[i] + 1, (r[i] - 1) + 1);

        if(f[i] < 0)
            f[i] = -1;
        else
        {
            ++f[i];
            g.Update(i + 1, f[i]);
        }
    }

    if(f[n + 1] == -1) cout << "-1";
    else cout << f[n + 1] - 1;
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    freopen("CDAY.INP", "r", stdin);
    freopen("CDAY.OUT", "w", stdout);

    Read();
    Solve();
}