#include <cstdio>
#include <algorithm>
using namespace std;

struct dddd{
    int x;
    int y;
}v[50050];

bool cmp( dddd a, dddd b ){
    return ( a.x < b.x );
}

int main()
{

    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);

    int n, i, j, s, t, d, k, a, b, mi, ma;

    i = 1;
    scanf("%d%d",&d,&n);
    for( i = 1; i <= n; ++i )
        scanf("%d%d",&v[i].x,&v[i].y);

    sort( v + 1, v + 1 + n, cmp );
    //for( i = 1; i <= n; ++i ) printf("%d %d\n",v[i].x,v[i].y);

    mi = -(1<<23);
    ma = -(1<<23);
    for( i = 1; i <= n; ++i ){
        if( v[i].x + v[i].y + mi > ma ){
            ma = v[i].x + v[i].y + mi;
        }
        if( v[i].y - v[i].x > mi ) mi = v[i].y - v[i].x;
    }

    printf("%d",ma);

    return 0;
}