#include<stdio.h>
#include<algorithm>
int const M=1000001,N=50001;
using namespace std;
struct strada{
    int d,l;
};
bool cmp(const strada a,const strada b){
    return a.d<b.d;
}
strada v[N];
int main(){
    int n,m,i,max=0,u=0,nr;
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&m,&n);
    for(i=1;i<=n;i++)
        scanf("%d%d",&v[i].d,&v[i].l);
    sort(&v[1],&v[n+1],cmp);
    for(i=1;i<=n;i++){
        nr=v[u].l+v[i].l+v[i].d-v[u].d;
        if(nr>max)
            max=nr;
        if(v[i].l>v[u].l+v[u].d){
            u=i;
            i=1;
        }
    }
    printf("%d",max);
    return 0;
}