#include <stdio.h>
#include <stdlib.h>

int v1[1000000], v2[1000000];

int main(){
    FILE *fin, *fout;
    int m, n, i, d, l, drum, drummax;

    fin = fopen ( "orase.in", "r");
    fout = fopen ( "orase.out", "w");
    fscanf( fin, "%d%d", &m, &n );
    for ( i = 0; i < n; i++){
        fscanf ( fin, "%d%d", &d, &l);
        if ( v1[d] == 0){
            v1[d] = l;
        }
        else {
            v2[d] = v1[d];
            v1[d] = l;
        }
    }
    drum = 0;
    drummax = 0;
    for (i = 0; i <= m; i++){
        if (v1[i] + drum > drummax)
            drummax = v1[i] + drum;
        if (v1[i] + v2[i] > drummax)
            drummax = v1[i] + v2[i];
        if (drum < v1[i])
            drum = v1[i];
        drum ++;
    }
    fprintf(fout, "%d", drummax);

    fclose(fin);
    fclose(fout);
    return 0;
}