#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

#define el '\n'
#define FIO ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;
typedef long double ld;
typedef complex<ld> pt;
typedef unsigned long long ull;

template<typename T, typename X>
using hashTable = gp_hash_table<T, X>;
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;

// mt19937_64 for long long
mt19937 rng(std::chrono::system_clock::now().time_since_epoch().count());

struct Node
{
    int mn = 1e9 + 5;
};

struct segTree
{
    vector<Node> tree;
    Node neutral = Node();

    segTree(int n)
    {
        int sz = 1;
        while(sz < n)
            sz *= 2;
        tree = vector<Node>(2 * sz);
    }

    Node single(int idx, int val)
    {
        Node ret;
        ret.mn = val;
        return ret;
    }

    Node merge(Node a, Node b)
    {
        Node ret;
        ret.mn = min(a.mn, b.mn);
        return ret;
    }

    void build(int node, int s, int e, vector<int> &v)
    {
        if(s == e)
        {
            tree[node] = single(s, v[s]);
            return;
        }
        int mid = (s + e) >> 1;
        build(2 * node, s, mid, v);
        build(2 * node + 1, mid + 1, e, v);
        tree[node] = merge(tree[2 * node], tree[2 * node + 1]);
    }

    Node query(int node, int s, int e, int l, int r)
    {
        if(r < s || l > e)
            return neutral;

        if(s >= l && e <= r)
            return tree[node];

        int mid = (s + e) >> 1;
        Node left = query(2 * node, s, mid, l, r);
        Node right = query(2 * node + 1, mid + 1, e, l, r);
        return merge(left, right);
    }

};

void doWork()
{
    int n, m;
    cin >> n >> m;
    vector<int> len(m + 1);
    vector<pair<int, int>> v(m + 1);

    for(int i = 1; i <= m; i++)
        cin >> v[i].first >> v[i].second, len[i] = v[i].second - v[i].first;

    segTree st(n + 1);
    vector<vector<int>> dp(m + 1, vector<int>(n + 1, 1e9 + 5));

    for(int i = 1; i + len[1] <= n; i++)
        dp[1][i] = abs(i - v[1].first);
    st.build(1, 1, n, dp[1]);

    for(int i = 2; i <= m; i++)
    {
        for(int j = 1, l, r; j + len[i] <= n; j++)
        {
            dp[i][j] = abs(v[i].first - j) +
                       st.query(1, 1, n, max(1, j - len[i - 1]), min(n, j + len[i])).mn;
        }
        st.build(1, 1, n, dp[i]);
    }
    cout << *min_element(dp[m].begin() + 1, dp[m].end());
}

signed main()
{
    FIO
    int T = 1;
//    cin >> T;
    for(int i = 1; i <= T; i++)
        doWork();
}