#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());

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;

    vector <vector<int>> dp(m + 1, vector<int>(n + 1));
    for(int i = 1; i <= m; i++)
    {
        for(int j = 1; j <= n; j++)
        {
            dp[i][j] = 1e9 + 5;

            for(int k = max(1, j - len[i - 1]); k <= min(n, j + len[i]); k++)
                dp[i][j] = min(dp[i][j], abs(v[i].first - j) + dp[i - 1][k]);
        }
    }

//    for(int i = 1; i <= m; i++, cout << '\n')
//        for(int j = 1; j <= n; j++)
//            cout << dp[i][j] << ' ';
    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();
}