#include<bits/stdc++.h>
#define int long long
using namespace std;
int n,k,num=1;
int sum[10005],f[10005];
struct node
{
int p,t;
bool operator < (const node &o)const
{
return p>o.p;
}
}a[10005];
signed main()
{
//	freopen("尼克的任务.in","w",stdin);
//	freopen("尼克的任务.out","r",stdout);
cin>>n>>k;
for(int i=1;i<=k;i++)
{
cin>>a[i].p>>a[i].t;
sum[a[i].p]++;
}
sort(a+1,a+k+1);
for(int i=n;i>=1;i--)
{
if(sum[i]==0)
f[i]=f[i+1]+1;
else for(int j=1;j<=sum[i];j++)
{
if(f[i+a[num].t]>f[i])
f[i]=f[i+a[num].t];
num++;
}
}
cout<<f[1]<<endl;
return 0;
}