#include <stdio.h>
#include <stdlib.h>
int str[1000001];
int main()
{
    int n, m, a, b, i, d, max;
    FILE *fi=fopen("orase.in", "r"), *fo=fopen("orase.out", "w");
    fscanf(fi, "%d%d", &m, &n);
    for(i=0;i<n;i++){
        fscanf(fi, "%d%d", &a, &b);
        if(str[a]<b)
            str[a]=b;
    }
    i=0;
    while(str[i]==0)
        i++;
    d=str[i];
    max=0;
    for(i=i+1;i<=n;i++){
        d++;
        if(str[i]!=0){
            if(d+str[i]>max)
                max=d+str[i];
            if(d<str[i])
                d=str[i];
        }
    }
    fprintf(fo, "%d", max);
    fclose(fi);
    fclose(fo);
    return 0;
}