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

struct orasele{int x,y;};
orasele v[50001];

bool compar(const orasele A, const orasele B){
    if(A.x==B.x)
        return A.y<B.y;
    return A.x<B.x;
}

int i,n,m,vscx,sc,scmax,vscy,pp,xinc,yinc;
int main(){
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&n,&m);
    for(i=1;i<=n;i++){
        scanf("%d%d",&v[i].x,&v[i].y);
    }
    sort(v+1,v+m+1,compar);
   xinc=v[1].x;
   yinc=v[1].y;
   sc=v[1].y;
   scmax=sc;
   for(i=2;i<=n;i++){
       pp=1;
       if(i!=n&&v[i].y>v[i].x-xinc+yinc){
           sc=v[i].y;
           xinc=v[i].x;
           yinc=v[i].y;
           pp=0;
        }
        else
            sc+=(v[i].x-xinc);
        if(sc>scmax&&pp==1)
                scmax=sc+v[i].y;
        else
        if(sc+v[i].y>scmax&&pp==0)
            scmax=sc;

   }
   printf("%d",scmax);
    return 0;
}