#include <bits/stdc++.h>
#define F first
#define S second
using namespace std;
using ld = long double;

struct stableSum {
    int cnt = 0;
    vector<ld> v, pref{0};
    void operator+=(ld a) {
        if (a < -1e-9) {
            cout << "-1\n"; exit(0);
        }
        int s = ++cnt;
        while (s % 2 == 0) {
            a += v.back();
            v.pop_back(), pref.pop_back();
            s /= 2;
        }
        v.push_back(a);
        pref.push_back(pref.back() + a);
    }
    ld val() { return pref.back(); }
};

void test_case() {
    cout << fixed << setprecision(12);
    int s, t, n;
    cin >> s >> t >> n;
    map<int, int, greater<int>> mp;
    for (int i = 0, m, h; i < n; ++i) {
        cin >> m >> h;
        int &x = mp[m];
        if (x == 0)
            x = h;
        else x = min(x, h);
    }
    vector<pair<int, int>> vp;
    for (auto p : mp)
        vp.push_back(p);
    if (s == t) {
        cout << "0\n"; return;
    }
    if (vp[0].F < s) {
        cout << "-1\n"; return;
    }
    stableSum ans;
    int mn = vp[0].S;
    for (int i = 1; i < vp.size(); ++i) {
        if (vp[i].F < t)
            break;
        if (vp[i].S >= mn)
            continue;
        ans += mn * log2(s * 1.0l / vp[i].F);
        mn = vp[i].S;
        s = vp[i].F;
    }
    ans += mn * log2(s * 1.0l / t);
    cout << ans.val() << '\n';
}

signed main() {
    ios::sync_with_stdio(false); cout.tie(nullptr); cin.tie(nullptr);
    test_case();
    // int tc; cin >> tc;
    // while (tc--) {
    //     test_case();
    // }
    return 0;
}