#include <bits/stdc++.h>

#define ll long long

using namespace std;

const ll N=;

ll total,f,n,ans;

struct re{

  ll a,b;

}a[N];

bool cmp(re x,re y)

{

  return(x.a<y.a);

}

double Random() { return rand()/(double) RAND_MAX ;}

/*ll judge(ll x)

{

  if (x<=0) return 0;

  ll xx=total-f*x,day=0,res=0;

  for (ll i=1;i<=n;i++)

  {

    if (log2(x)+log2(a[i].a)>log2(xx))

    {

      res+=xx/a[i].a;

      break;

    }

    ll num=min(xx/a[i].a/x,a[i].b-day+1);

    day+=num; xx-=num*a[i].a*x;

    res+=num*x;

  }

  ans=max(ans,res);

  return(res);

}*/

ll judge(ll times)

{

        if(times<=) return ;

        ll Money = total - times * f;

        ll res = , num, day = ;

        for(int i=;i<=n;i++)

        {

            num = min(Money / a[i].a / times, a[i].b - day + );

            Money -= num * a[i].a * times;

            day += num; res += times * num;

            if(day <= a[i].b)

            {

                num = Money / a[i].a;

                res += num;

                ans = max(ans, res);

                return res;

            }

        }

        ans = max(ans, res);

        return res;

}

void sa(double T)

{

  ll now=;

  while (T>=)

  {

    ll a=now+(ll)(T*(Random()*-));

    if (a<=) a=T*Random();

    ll de=judge(a)-judge(now);

    if (de>) now=a;

    T*=0.97;

  }

}

/*void sa(double T)

{

        ll Now = 1;

        while(T >= 1)

        {

            ll A = Now + (ll)(T * (Random()*2-1)) ;

            if(A<=0) A = T*Random();

            ll dE = judge(A) - judge(Now);

            if(dE > 0)

                Now = A;

            T *= 0.97;

        }

}*/

int main()

{

  srand(time()^size_t(new char));

  freopen("1.in","r",stdin);

  freopen("1.out","w",stdout);

  ios::sync_with_stdio(false);

  while (cin>>total>>f>>n)

  {

    ans=;

    for(ll i=;i<=n;i++)

      cin>>a[i].a>>a[i].b;

    sort(a+,a+n+,cmp);

    ll m=n,d=-;

    n=;

    for (ll i=;i<=m;i++)

      if (a[i].b>d) a[++n]=a[i],d=a[i].b;

    sa(total/f+);

    cout<<ans<<endl;

  }

  return ;

}