#include <bits/stdc++.h>

#define f first
#define s second
#define pb push_back
#define mp make_pair

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int N = 300500, inf = 1e9, mod = 998244353;
const ll INF = 1e18;

int sum(int a, int b) {
    a += b;
    if (a >= mod)
        a -= mod;
    return a;
}

int sub(int a, int b) {
    a -= b;
    if (a < 0)
        a += mod;
    return a;
}

int mult(int a, int b) {
    return 1ll * a * b % mod;
}

int bp(int a, int b) {
    int res = 1;
    while (b) {
        if (b & 1)
            res = mult(res, a);
        a = mult(a, a);
        b >>= 1;
    }
    return res;
}

int inv(int x) {
    return bp(x, mod - 2);
}
int s, t;
int n;
typedef long double ld;
void solve() {
    cin >> s >> t >> n;
    vector<pair<int,int>> ev;
    int st = 1e9;
    for (int i = 1; i <= n; i++) {
        int m, h;
        cin >> m >> h;
        if (m < t) {
            continue;
        }
        if (m >= s) {
            st = min(st, h);
            continue;
        }
        ev.emplace_back(m, h);
    }
    sort(ev.begin(), ev.end());
    reverse(ev.begin(), ev.end());
    int lst = s;
    ld ans = 0;
    if (st > 1e8) {
        cout << -1;
        return ;
    }
    for (int z = 0; z < ev.size(); z++) {
        if (ev[z].first < lst) {
            ans += log2l((ld)lst / ev[z].first) * st;
            lst = ev[z].first;
        }
        st = min(st, ev[z].second);
    }
    ans += log2l((ld)lst / t) * st;
    cout << fixed << setprecision(20) << ans;

}

int main() {
#ifdef DEBUG
    freopen("input.txt", "r", stdin);
#endif
    ios_base::sync_with_stdio(false);
    int t = 1;
//    cin >> t;
    for (int i = 1; i <= t; i++) {
//        cout << "Case #" << i << endl;
        solve();
    }
}