#include<iostream>
#include<vector>
#include<algorithm>
#define x first
#define y second
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
const int N = 100010;
PII p[N];
LL n, len;
vector<PII> v;
bool check(int mid)
{
    int l = 0, r = 0;
    for (int i = 0; i < n; ++ i)
    {
        if (p[i].y <= mid)
        {
            l = p[i].x - (mid - p[i].y);
            r = p[i].x + (mid - p[i].y);
            v.push_back({ l,r });
        }
    }
    sort(v.begin(), v.end());
    if (v.size())
    {
        int st = v[0].x, ed = v[0].y;
        for (int i = 0; i < v.size(); ++ i)
        {
            if (v[i].x <= ed + 1) ed = max(ed, v[i].y);
            else return false;
        }
        if (st <= 1 && ed >= len) return true;
        else return false;
    }
    else return false;
}
int main()
{
    cin >> n >> len;
    for (int i = 0; i < n; ++ i)
    {
        int a, b;
        scanf("%d%d", &a, &b);
        p[i] = { a,b };
    }
    int l = 0, r = 2e9;
    while (l < r)
    {
        if (v.size()) v.clear();
        int mid = l + r >> 1;
        if (check(mid)) r = mid;
        else l = mid + 1;
    }
    cout << r << endl;
    return 0;
}