#include<fstream>
#include<algorithm>
using namespace std;
int m, n, i, d[1000003], sol;
struct cub{
    int d;
    int sum;
};
cub x[1000003];
int maxim(int x, int y){
    return ( x > y ? x : y );
}
int cmp(cub x, cub y){
    return x.d<y.d;
}
ifstream in("orase.in");
ofstream out("orase.out");
int main(){
    in>>m>>n;
    for(i=1; i<=n; i++){
        in>>x[i].d>>x[i].sum;
    }
    sort(x+1, x+n+1, cmp);
    for(i=2; i<=n; i++){
        d[i]=maxim(d[i-1]+x[i].d-x[i-1].d, x[i-1].sum+x[i].d-x[i-1].d);
    }
    for(i=2; i<=n; i++){
        if(d[i]+x[i].sum>sol)
            sol=d[i]+x[i].sum;
    }
    out<<sol;
return 0;
}