#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;

const ld TWO = 2.0;

signed main() {
	ll cur_size, target_size, n;
	cin >> cur_size >> target_size >> n;

	vector<pair<ll, ll>> blades(n); // Avg Size Req, Time
	for (ll i = 0; i < n; i++) {
		cin >> blades[i].first >> blades[i].second;
	}

	sort(blades.begin(), blades.end());
	reverse(blades.begin(), blades.end());

	ll T = 1e9;
	ll i = 0;
	for (; i < n && cur_size <= blades[i].first; i++) {
		T = min(T, blades[i].second);
	}

	if (i == 0) {
		cout << -1 << "\n";
		return 0;
	}

	ld ans = 0.0;
	for (; i < n && blades[i].first > target_size; i++) {
		ld t = ((log((ld)cur_size) - log((ld)blades[i].first)) / log(TWO)) * ((ld)T);
		ans += t;
		cur_size = blades[i].first;
		for (; i < n && cur_size <= blades[i].first; i++) {
			T = min(T, blades[i].second);
		}
	}

	ld t = ((log((ld)cur_size) - log((ld)target_size)) / log(TWO)) * ((ld)T);
	ans += t;

	cout << setprecision(20);
	cout << ans << endl;
}