#include <stdio.h>
using namespace std;

#define in "orase.in"
#define out "orase.out"
#define dim 50001

int N, M;
int D1[dim], L1[dim];
int D2[dim], L2[dim];

int main()
{
    freopen(in,"r",stdin);
    freopen(out,"w",stdout);
    
    scanf("%d%d", &M, &N);
    for ( int i = 1; i <= N; i++ )
        scanf("%d%d", &D1[i], &L1[i]), D2[i] = D1[i], L2[i] = L1[i];
        
    // N*N 
    int q, maxim=-1;
    for ( int i = 1; i < N; i++ )
        for ( int j = i+1; j <= N; j++ )
        {
            if ( D2[i] < D2[j] ) q = D2[j]-D2[i];
             else                    q = D2[i]-D2[j];
             
             if ( maxim < L2[i] + L2[j] + q ) maxim = L2[i] + L2[j] + q; 
        }
            
    
    printf("%d", maxim);
}