#include <bits/stdc++.h>
#define fi first
#define se second
#define bit(x) (1LL << (x))
#define getbit(x, i) (((x) >> (i)) & 1)
#define ALL(x) (x).begin(), (x).end()
using namespace std;

template <typename T1, typename T2> bool maxi(T1 &a, T2 b) {
    if (a < b) {a = b; return true;} return false;
}

template <typename T1, typename T2> bool mini(T1 &a, T2 b) {
    if (a > b) {a = b; return true;} return false;
}

const int N = 2e5 + 5;
const int oo = 1e9;
const long long ooo = 1e18;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);  
    int s, t, n;
    cin >> s >> t >> n;
    vector <pair <int, int>> a(n);
    for (auto &[m, h] : a) {
        cin >> m >> h;
    }

    sort(ALL(a));
    int cur = s;
    int minh = oo;
    double ans = 0;
    if (a[n - 1].fi < cur) {
        cout << "-1\n";
        return 0;
    }

    for (int i = n - 1; i >= 0; i--) {
        if (a[i].fi >= cur) {
            mini(minh, a[i].se);
            continue;
        }

        int nex = max(a[i].fi, t);
        double x = log(2) / minh;
        ans += - log((double) nex/cur) / x;
        mini(minh, a[i].se);
        cur = nex;
        // cerr << a[i].fi << " " << t << " " << cur << " " << nex << "\n";
        if (nex == t) 
            break;
    }

    if (cur > t) {
        double a = log(2) / minh;
        ans += - log((double) t/cur) / a;
    }

    cout << fixed << setprecision(9) << ans << "\n";
}