#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
int main(){
	ios::sync_with_stdio(false);
	cin.tie(0),cout.tie(0);	
	
	i64 s,t,n;
	cin>>s>>t>>n;

	set<array<i64,2>> bl;

	for(int i=1;i<=n;i++){
		i64 m,h;
		cin>>m>>h;

		auto hav=bl.upper_bound({m,h});
		if(hav!=bl.end()&&(*hav)[1]<=h) continue;

		auto it=hav;
		while(it!=bl.begin() && (*prev(it))[1]>h){
			bl.erase(it,hav);
			it--;
		}
		bl.insert({m,h});
	}

	auto m=bl.upper_bound({s,0});
	if(m==bl.end()){
		cout<<"-1\n";
		return 0;
	}

	long double now=t,ans=0;
	auto p=bl.upper_bound({t,0});
	while(((*p)[0])<s){
		ans+=(*p)[1]*log2((*p)[0]/now);
		now=(*p)[0];
		p++;
	}
	ans+=(*p)[1]*log2(s/now);
	cout<<fixed<<setprecision(10)<<ans<<'\n';
	
	return 0;
}