#include <bits/stdc++.h>

using namespace std;

ifstream in("orase.in");
ofstream out("orase.out");

vector<pair<int,int>> r;
int M,N;

int calculation_distantiation(pair<int,int> a, pair<int,int> b)
{
    return a.second + abs(a.first - b.first) + b.second;
}

int main()
{
    in>>M>>N;
    pair<int,int> pmax = make_pair(-1,-1);
    int dmax = -1;
    for(int i = 0; i < M;++i)
    {
        int x, y;
        in>>x>>y;
        if(y > pmax.second)
            pmax = make_pair(x,y);

        r.push_back(make_pair(x,y));
    }
    for(auto& e : r)
    {
        dmax = max(calculation_distantiation(e,pmax),dmax);
    }
    out<<dmax;
    return 0;
}