photo

#include <iostream>
#include <cstdio>

#include <vector>
#include <set>

#include <utility>
#include <cstring>

using namespace std;

#define REP(i, n) for (int i = 0, _n = (n); i < _n; ++i)

const int MAX_N = int(2e5) + 20, MAX_M = int(1e5) + 20;

int N, M;
pair<int, int> I[MAX_M];

struct CompareInterval {
    bool operator()(int i, int j) {
        return I[i] < I[j];
    }
};

int main() {
    freopen("photo.in", "r", stdin);
    freopen("photo.out", "w", stdout);

    cin >> N >> M;
    REP(i, M) {
        int a, b; cin >> a >> b;
        I[i] = make_pair(a - 1, b);
    }

    static vector<int> ev[MAX_N];
    REP(i, M) {
        ev[I[i].first].push_back(i + 1);
        ev[I[i].second].push_back(-i - 1);
    }

    static int dp[MAX_N];
    memset(dp, -1, sizeof(dp));

    dp[0] = 0;
    set<int, CompareInterval> S;
    int start = -1;
    REP(i, N) {
        REP(j, ev[i].size())
            if (ev[i][j] > 0) S.insert(ev[i][j] - 1);
            else {
                int c = -ev[i][j] - 1;
                S.erase(c);
                start = max(start, I[c].first);
            }
        if (S.empty() || I[*S.rbegin()].first != i)
            dp[i + 1] = dp[i];
        int l = S.empty() ? i : I[*S.begin()].first;
        if (l > start && dp[l] != -1)
            dp[i + 1] = max(dp[i + 1], dp[l] + 1);
    }

    cout << dp[N] << "\n";
    return 0;
}