#include<cstdio>
#include<algorithm>

struct oras{
    int d;
    int l;
}v[50010];

bool cresc(oras x,oras y){

    return x.d<y.d;

}

int main(){

    int max=0,mult,i,j,q,p,m,n;

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

    scanf("%d%d\n",&m,&n);
    for(i=1;i<=n;i++){
        scanf("%d%d",&v[i].d,&v[i].l);
    }
    std::sort(&v[1],&v[n+1],cresc);
    for(i=1;i<=n;i++){
        for(j=n;j>=i;j--){
            mult=v[i].l+v[j].l+v[j].d-v[i].d;
            if(mult>max){
                max=mult;
                q=i;
                p=j;
            }
        }
    }
    printf("%d",v[q].l+v[p].l+v[p].d-v[q].d);
    return 0;

}