//
//  main.cpp
//

#include <algorithm>
#include <array>
#include <assert.h>
#include <functional>
#include <iostream>
#include <iomanip>
#include <limits>
#include <map>
#include <math.h>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

using ll = long long;
using ull = unsigned long long;

template<typename T>
vector<T> make_vec_nd(T init, size_t size) {
    return vector<T>(size, init);
}
template<typename T, typename... Args>
auto make_vec_nd(T init, size_t size, Args... rest) {
    auto inner = make_vec_nd(init, rest...);
    return vector<decltype(inner)>(size, inner);
}

#define rep(i,a,b) for(ll i=(a);i<(b);i++)
#define rrep(i,a,b) for(ll i=(a)-1;i>=(b);i--)

template<typename Monoid>
struct SegmentTree {
    using Op = function<Monoid(const Monoid& lhs, const Monoid& rhs)>;
    size_t width;
    vector<Monoid> node;
    Op op;
    Monoid unit;
    
    SegmentTree(const vector<Monoid>& elems, Op op, Monoid unit)
    : op(op)
    , unit(unit) {
        width = 1;
        while (width < elems.size()) {
            width *= 2;
        }
        node.resize(2 * width - 1, unit);
        for (size_t i = 0; i < elems.size(); i++) {
            node[i + width - 1] = elems[i];
        }
        for (size_t i = width - 2; i != numeric_limits<size_t>::max(); i--) {
            node[i] = op(node[2 * i + 1], node[2 * i + 2]);
        }
    }
    void update(size_t idx, Monoid elem) {
        idx += width - 1;
        node[idx] = elem;
        while (idx >= 1) {
            idx = (idx - 1) / 2;
            node[idx] = op(node[2 * idx + 1], node[2 * idx + 2]);
        }
    }
    Monoid query(size_t a, size_t b, size_t root = 0) const {
        assert(0 <= a && b <= width);
        size_t left = root, right = root;
        while (left < width - 1) {
            left = 2 * left + 1;
        }
        left -= (width - 1);
        while (right < width - 1) {
            right = 2 * right + 2;
        }
        right -= (width - 1);
        right++;
        if (right <= a || b <= left) {
            return unit;
        }
        if (a <= left && right <= b) {
            return node[root];
        }
        Monoid ans_l = query(a, b, 2 * root + 1);
        Monoid ans_r = query(a, b, 2 * root + 2);
        return op(ans_l, ans_r);
    }
};

int main() {
    ll N, M;
    cin >> N >> M;
    using Range = pair<ll, ll>;
    unordered_map<ll, Range> right_to_range;
    rep(i,0,M) {
        ll a, b;
        cin >> a >> b;
        a--;
        b--;
        if (right_to_range.find(b) != right_to_range.end()) {
            Range elem = right_to_range[b];
            if (b-a < elem.second - elem.first) {
                right_to_range[b] = {a, b};
            }
        } else {
            right_to_range[b] = {a, b};
        }
    }
    vector<Range> ranges_pre;
    rep(i,0,N) {
        if (right_to_range.find(i) != right_to_range.end()) {
            ranges_pre.push_back(right_to_range[i]);
        }
    }
    vector<Range> ranges;
    ranges.push_back(ranges_pre[0]);
    rep(i,1,ranges_pre.size()) {
        if (ranges_pre[i].first > ranges.back().first) {
            ranges.push_back(ranges_pre[i]);
        }
    }
    right_to_range.clear();
    rep(i,0,ranges.size()) {
        right_to_range[ranges[i].second] = ranges[i];
    }
    vector<ll> elems(N-1, numeric_limits<ll>::max());
    SegmentTree<ll> dp(elems,
                       [](ll lhs, ll rhs){ return min(lhs, rhs); },
                       numeric_limits<ll>::max());
    if (right_to_range.find(1) != right_to_range.end()) {
        dp.update(0, 1);
    } else {
        dp.update(0, 0);
    }
    rep(i,1,N-1) {
        if (right_to_range.find(i+1) != right_to_range.end()) {
            Range added = right_to_range[i+1];
            ll len = added.second  - added.first;
            ll mn = dp.query(max((ll)0, i-len), i);
            dp.update(i, mn + 1);
        } else {
            dp.update(i, dp.query(i-1, i));
        }
    }
    cout << dp.query(N-2, N-1) << endl;
}


Measured memory_bytes: 9112