#include <bits/stdc++.h>

using namespace std;

using ll = long long;

#define all(x) x.begin(), x.end()

struct sparse {
    vector<vector<int>> sp;
    vector<int> pw;

    sparse(vector<int> a) {
        int n = a.size();
        int k = 1;
        while ((1 << (k - 1)) < n)
            ++k;
        sp.resize(k, vector<int> (n));
        sp[0] = a;
        for (int j = 1; j < k; ++j) {
            for (int i = 0; i + (1 << (j - 1)) < n; ++i)
                sp[j][i] = min(sp[j - 1][i], sp[j - 1][i + (1 << (j - 1))]);
        }
        pw.resize(n + 1);
        for (int i = 2; i <= n; ++i)
            pw[i] = pw[i / 2] + 1;
    }

    int get(int l, int r) {
        int k = pw[r - l];
        // cout << k << ' ' << l << ' ' << r << ' ' << sp[0].size() << ' ' << sp.size() << '\n';
        return min(sp[k][l], sp[k][r - (1 << k)]);
    }
};

int main() {
    std::ios_base::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);

    int n, m;
    cin >> n >> m;
    vector<pair<int, int>> a(m);
    for (int i = 0; i < m; ++i) {
        cin >> a[i].first >> a[i].second;
        --a[i].first, --a[i].second;
    }
    const int inf = 1e9 + 228;
    vector<int> dp(n, 0);
    for (int i = 0; i < n; ++i) {
        dp[i] = abs(i - a[0].first);
    }
    for (int j = 1; j < m; ++j) {
        sparse t(dp);
        vector<int> _dp(n, inf);
        // for (int i : dp)
        //  cout << i << ' ';
        // cout << '\n';
        int s1 = a[j].second - a[j].first + 1;
        int s2 = a[j - 1].second - a[j - 1].first;
        for (int i = 0; i < n; ++i) {
            int tmp = i - s2;
            if (tmp >= 0) {
                _dp[i] = min(_dp[i], t.get(tmp, i + 1));
            } else {
                _dp[i] = min(_dp[i], t.get(0, i + 1));
                _dp[i] = min(_dp[i], t.get(n + tmp, n));
            }
            int k = i + s1;
            if (k <= n) {
                _dp[i] = min(_dp[i], t.get(i, k));
            } else {
                _dp[i] = min(_dp[i], t.get(i, n));
                _dp[i] = min(_dp[i], t.get(0, k - n));
            }
            _dp[i] += abs(i - a[j].first);
        }
        dp = _dp;
    }
    cout << *min_element(all(dp)) << '\n';

    return 0;
}