#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>

using namespace std;
using ld = long double;

using pld = pair<ld, ld>;

const ld EPS = 1e-8;

ld compute(ld a, ld b, ld h) {
	return -1*h*log2(b/a);
}

int main() {
	ld s, t; int n;
	cin >> s >> t >> n;
	vector<pld> b(n);
	for (int i = 0; i < n; i++) {
		cin >> b[i].first >> b[i].second;
	}
	sort(b.begin(), b.end());
	reverse(b.begin(), b.end());
	//for (auto x : b) cout << x.first << ' ' << x.second << endl;
	int back = 0;
	while (b[back].first > t - EPS) back++;
	int p = -1;
	ld bestspeed = 0;
	for (int i = 0; i < back; i++) {
		if (b[i].first < s - EPS) {
			break;
		}
		if (b[i].second > bestspeed) {
			bestspeed = b[i].second;
			p = i;
		}
	}
	if (p < 0) {
		cout << -1 << endl;
		return 0;
	}
	int p2 = p+1;
	ld ans = 0;
	while (p2 < back) {
		if (b[p2].second < b[p].second) {
			ans += compute(s, b[p2].first, b[p].second);
			p = p2;
			s = b[p2].first;
		}
		p2++;
	}
	ans += compute(s, t, b[p].second);
	cout << fixed << setprecision(10);
	cout << ans << endl;
}