#include <iostream>
#include <cstdio>

using namespace std;
/*int abs(int k,int l)
{
    int dif=k-l;
    if(dif<0)
        dif=0-dif;
    return dif;
}
int strazil[100001],strazip[100001];
int sol(int r1,int r2){
   int r=abs(strazip[r1],strazip[r2])+strazil[r1]+strazil[r2];
   return r;
};*/
int d[10001],p[10001];
int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
//    int n,c,i,r1,r=0,j;
    int t,a=0,sol=0,n,c,i;
    scanf("%d%d", &n,&c);
    for(i=1;i<=c;i++)
    {
        scanf("%d%d", &p[i],&d[i]);
        t=a+p[i]+d[i];
        if(t>sol)
            sol=t;
        if(d[i]-p[i]>a)
            a=d[i]-p[i];
    }
    printf("%d", sol);
 /*   for(i=1;i<=n-1;i++)
    {
        for(j=i+1;j<=n;j++)
        {
            r1=sol(i,j);
            if(r1>r)
                r=r1;
        }
    }
    printf("%d", r);*/
    return 0;
}