#pragma GCC optimize("03")
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <fstream>
#include <string>
#include <set>
#include <deque>
#include <queue>
#include <map>
#include <bitset>
#include <random>
#include <cassert>
#include <unordered_map>
#include <unordered_set>
#include <math.h>
#include <chrono>
#include <ctime>
using namespace std;
using namespace chrono;
#define all(a) a.begin(), a.end()
#define allr(a) a.rbegin(), a.rend()
#define endl "\n"
mt19937 rnd(std::chrono::high_resolution_clock::now().time_since_epoch().count());
typedef long long ll;
typedef long double ld;
struct Tree {
    const ll inf = 1e18;
    vector<ll> tree;
    map<ll, ll> posit;
    void init(ll n) {
        tree.resize(4 * n, inf);
    }
    void update(ll v, ll tl, ll tr, ll pos, ll add) {
        if (tl == tr) {
            tree[v] = add;
            posit[tl] = v;
            return;
        }
        ll mid = (tl + tr) >> 1;
        if (pos <= mid) {
            update(v << 1, tl, mid, pos, add);
        }
        else {
            update((v << 1) | 1, mid + 1, tr, pos, add);
        }
        tree[v] = min(tree[v << 1], tree[(v << 1) | 1]);
    }
    ll get_min(ll v, ll tl, ll tr, ll l, ll r) {
        if (tr<l || tl>r) {
            return inf;
        }
        if (tl >= l && tr <= r) {
            return tree[v];
        }
        ll mid = (tl + tr) >> 1;
        return min(get_min(v << 1, tl, mid, l, r), get_min((v<<1)|1, mid + 1, tr, l, r));
    }
    void update_el(ll n, ll pos, ll add) {
        update(1, 0, n - 1, pos, add);
    }
    ll qet_mn_seg(ll n, ll l, ll r) {
        return get_min(1, 0, n - 1, l, r);
    }
};
signed main() {
#ifdef _DEBUG
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    srand(time(NULL));
    ll n, m;
    cin >> n >> m;
    vector<pair<ll, ll>> a(m);
    vector<Tree> dp(m);
    for (ll i = 0; i < m; i++) {
        cin >> a[i].first >> a[i].second;
        a[i].first--, a[i].second--;
        dp[i].init(n);
    }
    for (ll i = 0; i < m; i++) {
        for (ll j = 0; j < n; j++) {
            if (i > 0) {
                if (i == 1) {
                    cout << "";
                }
                ll len = a[i].second - a[i].first + (ll)1;
                ll len2 = a[i - 1].second - a[i - 1].first + 1;
                if (j - len + 1 < 0) {
                    continue;
                }
                ll l = max(len2 - 1, j - len + 1), r = min(n - 1, j + len2 - 1);
                ll cur_min = dp[i - 1].qet_mn_seg(n, l, r);
                dp[i].update_el(n, j, cur_min + abs(a[i].second - j));
            }
            else {
                dp[i].update_el(n, j, abs(a[i].second - j));
            }
        }
    }
    ll ans = dp.back().tree[1];
    cout << ans << endl;
}