// File generated on 24/11/13 22:42 by Anand

#include <bits/stdc++.h>
using namespace std;

#define fastio ios::sync_with_stdio(false); cin.tie(0)
#define rep(i, m, n)  for (long long i = m; i < n; i++)
#define DEBUG(x) cout << "debug: " << x << "\n"
#define YES cout << "YES\n"
#define NO cout << "NO\n"
#define pb push_back
#define sz size

typedef long long ll;
typedef pair<ll,ll> pll;
typedef vector<ll> vec;
typedef vector<vec> vecc;
typedef vector<pll> vecp;

void SOLVE() {
   ll s, t, n;
   cin >> s >> t >> n;

   vecp a(n);
   rep(i,0,n) {
        ll x, y;
        cin >> x >> y;
        a[i] = {-x, y};
   }

   sort(a.begin(), a.end());

   ll mi = 1e7 + 5;

   vecp g;

   rep(i,0,n) {
        if (mi > a[i].second) {mi = a[i].second; g.pb({-a[i].first, mi});}
   }

   long double ans = 0;

   g.pb({0, 0});

   n = g.size();

    rep(i,0,n) {
        if (s > g[0].first) {cout << "-1\n"; return;}
        else if (i + 1 < n && s <= g[i + 1].first) continue;
        else if (i + 1 < n && t >= g[i + 1].first) {ans += g[i].second * (log2(s) - log2(t)); s = t; break;}
        else if (i + 1 < n) {ans += g[i].second * (log2(s) - log2(g[i+1].first)); s = g[i + 1].first;}
        else {cout << "-1\n"; return;}
    }

    cout << setprecision(8) << ans << "\n";
}

signed main() {
    fastio;

		SOLVE();

    return 0;
}