///Enta etfsh5t nseet el rank

#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,fma")

#include "bits/stdc++.h"
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp> // Including tree_order_statistics_node_update

using namespace std;
using namespace __gnu_pbds;

template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,
        tree_order_statistics_node_update>;
#define pb push_back
#define F first
#define S second
#define f(i, a, b) for (int i = a; i < b; i++)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define sz(x) (int)(x).size()
//#define mp(x, y) make_pair(x, y)
#define popCnt(x) (__builtin_popcountll(x))
#define int ll

using ll = long long;
using ull = unsigned long long;
using uint = uint32_t;
using ii = pair<int, int>;

const int N = 3e6 + 6, A = 26, LG = 18, MOD = (119 << 23) + 1;
const long double PI = acos(-1);
const long double EPS = 1e-9;
const ll INF = 1e15;
int n, m;
int s[N], t[N];

void doWork() {

    cin >> n >> m;
    f(i, 1, m + 1) cin >> s[i] >> t[i];
    vector<vector<int>> dp(m + 1, vector<int>(n + 1, 1e9));
    f(i, 1, n + 1)dp[0][i] = 0;
    f(i, 1, m + 1) {
        multiset<int> st;
        for (int j = 1; j <= t[i] - s[i]; j++)
            st.insert(dp[i - 1][j]);
        for (int j = 1; j <= n; j++) {
            int a = t[i] - s[i], b = t[i - 1] - s[i - 1];
            if (j - b > 1)
                st.erase(st.find(dp[i - 1][j - b - 1]));
            if (j + a <= n)
                st.insert(dp[i - 1][j + a]);
            dp[i][j] = abs(s[i] - j) + *st.begin();
        }
    }

    cout << *min_element(all(dp[m])) << '\n';

}


int32_t main() {
#ifdef ONLINE_JUDGE
    ios_base::sync_with_stdio(0);
    cin.tie(0);
#endif // ONLINE_JUDGE
    int t = 1;
//    cin >> t;
    while (t--)
        doWork();

    return 0;
}