#include <bits/stdc++.h>

using namespace std;

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

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

int main()
{
    in>>M>>N;
    int ipm = 0, dmax = -1;
    for(int i = 0; i < M; ++i)
    {
        int x, y;in>>x>>y;
        r.push_back(make_pair(x,y));
        if(x+y>dmax)
        {
            dmax = x+y;
            ipm = i;
        }
    }
    for(int i = 0; i < M; ++i)
    {
        dmax  = max(dmax,r[i].second + abs(r[i].first - r[ipm].first) + r[ipm].second);
    }
    out<<dmax;
    return 0;
}