#include<bits/stdc++.h>
using namespace std;
struct node{int x,y,L;}a[11000];
bool cmp(node n1,node n2){return n1.x<n2.x;}
int f[11000];
int main()
{
    int T,n;scanf("%d%d",&T,&n);
    for (int i=1;i<=n;i++)scanf("%d%d",&a[i].x,&a[i].L),a[i].y=a[i].x+a[i].L-1;
    sort(a+1,a+n+1,cmp);
    memset(f,63,sizeof(f));f[T+1]=0;
    int j=n;
    for(int i=T;i>=1;i--)
    {
        bool bk=0;
        for(;j>=1;j--)
        {
            if(a[j].x==i)f[i]=min(f[i],f[a[j].y+1]+a[j].L),bk=1;
            if(a[j].x<i)break;
        }
        if(bk==0)f[i]=f[i+1];
    }
    printf("%d\n",T-f[1]);
    return 0;
}