#include <bits/stdc++.h>
using namespace std;
double nxt(double size, double want, double div2){
    return (log2(size) - log2(want)) * div2;
}
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    //sort by t
    double s, t;int n; cin >> s  >> t >> n;
    pair<double, double> process[n];
    double mxblade = 0;
    for(int i = 0; i < n; i++){ //blade size, hour
        cin >> process[i].first >> process[i].second;
        mxblade = max(mxblade, process[i].first);
    }
    sort(process, process+n);
    double ret = 0;
    
    int curSpeed = process[n-1].second, ind = n-1;
    bool can = true;
    if(mxblade < s){
        can = false;
    }
    while(s > t && can){
        while(ind >= 0 && s <= process[ind].first){
            curSpeed = min(curSpeed, (int)process[ind].second);
            ind--;
        }
        int goal = t;
        if(ind!=-1)goal= max(t, process[ind].first);
        ret += nxt(s, goal, curSpeed);
        s = goal;
    }

    if(can)cout << fixed << setprecision(10) << ret << "\n";
    else cout << -1 << "\n";
}