#include <stdio.h>
#include <stdlib.h>
#include <algorithm>
#define MAXN 1000000

#define BUF_SIZE 16384
char buf[BUF_SIZE];
int pbuf=BUF_SIZE;
FILE*fi,*fo;
inline char nextch(){
    if(pbuf==BUF_SIZE){
        fread(buf, BUF_SIZE, 1, fi);
        pbuf=0;
    }
    return buf[pbuf++];
}
inline long long nextnum(){
    long long a=0;
    char c=nextch();
    while((c<'0' || c>'9') && c!='-')
        c=nextch();
    int semn=1;
    if(c=='-'){
        semn=-1;
        c=nextch();
    }
    while('0'<=c && c<='9'){
        a=a*10+c-'0';
        c=nextch();
    }
    return a*semn;
}

struct linie{
    int d, l;
} v[50000];

int cmp(linie A, linie B){
    if(A.d<B.d)
        return 1;
    return 0;
}

inline int maxim(int a, int b){
    return a > b ? a : b;
}

int main(){
    fi=fopen("orase.in","r");
    fo=fopen("orase.out","w");
    int m=nextnum(), n=nextnum();
    for(int i=0;i<n;i++){
        v[i].d=nextnum();
        v[i].l=nextnum();
    }
    std::sort(v, v+n, cmp);
    int d=2*v[0].l;
    int max=-1;
    for(int i=1;i<n;i++){
        d=maxim(d+(v[i].l-v[i-1].l)+(v[i].d-v[i-1].d), v[i-1].l+v[i].l+v[i].d-v[i-1].d);
        //printf("%d ", d);
        if(d>max)
            max=d;
    }
    fprintf(fo,"%d", max);
    fclose(fi);
    fclose(fo);
    return 0;
}