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

int v[1000001];

int main()
{
    int n,k,i,nr,m,p,max=0;
    FILE*fi,*fo;
    fi=fopen("orase.in","r");
    fo=fopen("orase.out","w");
    fscanf(fi,"%d%d",&k,&n);
    for(i=0; i<n; i++)
    {
        fscanf(fi,"%d%d",&m,&p);
        if(v[m]<p)
            v[m]=p;
    }
    nr=0;
    for(i=1; i<=k; i++)
    {
        if((i-nr)+v[nr]+v[i]>max)
            max=(i-nr)+v[nr]+v[i];
        if(v[i]+nr>v[nr]+i)
            nr=i;
    }
    fprintf(fo,"%d",max);
    fclose(fi);
    fclose(fo);
    return 0;
}