#include<stdio.h>
#include<algorithm>
using namespace std;

struct nod {int l,d;}v[50001];
int n,m,i,j,k,l;
long long int lmax,sol,s1;

int cmp(nod a,nod b){
return a.d<b.d;}

int main(){
FILE *f=fopen("orase.in","r");
fscanf(f,"%d%d",&n,&n);

for(i=1;i<=n;i++)
        fscanf(f,"%d%d",&v[i].d,&v[i].l);
fclose(f);

sort(v+1,v+1+n,cmp);
lmax=v[1].l;
for(i=2;i<=n;i++)
        {lmax+=v[i].d-v[i-1].d;
        s1=lmax + v[i].l;
        if(s1>sol)
                sol=s1;
       if(v[i].l>lmax)lmax=v[i].l;
        }

FILE *g=fopen("orase.out","w");
fprintf(g,"%d",sol);
fclose(g);
return 0;}