#include<bits/stdc++.h>
using namespace std;
inline void debug(){
    #ifndef mrclr
    freopen("C:/Users/52419/Desktop/icpc/debug.in", "r", stdin);
    #endif
}

const int N = 1e5 + 10;
int n; double s, t, ans;
pair<double, double> b[N];

#define gc getchar()
#define in read()
inline int read(){
    int f=1,k=0; char cp=gc; while(cp!='-'&&(cp<'0'||cp>'9')) cp=gc; if(cp=='-') f=-1,cp=gc;
    while(cp>='0'&&cp<='9') k=(k<<3)+(k<<1)+cp-48,cp=gc; return f*k;
}
int main(){ //debug();
    s = in, t = in, n = in;
    for(int i = 1; i <= n; i++) b[i].first = in, b[i].second = in;
    sort(b + 1, b + n + 1, [](pair<int, int> x, pair<int, int> y){
        return x.first == y.first? x.second < y.second : x.first > y.first;
    });
    if(s > b[1].first){ cout<< -1 <<endl; return 0; }
    double speed = b[1].second; int pos = 1;
    while(s != t){
        while(pos != n && b[pos + 1].first >= s) pos++, speed = min(speed, b[pos].second);
        double target = pos == n? t : b[pos + 1].first;
        ans += speed * log2(s / target); s = target; 
    } printf("%.8f\n", ans);

    return 0;
}