import math

class blade():
   def __init__(self, max_, speed):
        self.max_ = max_
        self.speed = speed

arr_1 = [int(i) for i in  input().split()]
s,t,n = arr_1[0],arr_1[1],arr_1[2]

blades = []
blades.append(blade(0,0))


for i in range(0,n):
    arr_2 = [int(i) for i in  input().split()]
    blade_ = blade(arr_2[0],arr_2[1])
    blades.append(blade_)


blades.sort(key=lambda x: (x.max_, -x.speed))


    

if(blades[len(blades)-1].max_ < s):
    print(-1)
else:
    time = 0
    c_b_i = len(blades)-1
    cb = blades[c_b_i]
    while(abs(s-t)>10**(-5)):
        # haben wir das opt. blade für größre
        i = 1

        
        while(blades[c_b_i-i].max_ >= s):
            if(blades[c_b_i-i].speed < cb.speed):
                c_b_i = c_b_i-i   
                cb = blades[c_b_i]
                break
            i += 1
        #bis nächsten blade schneiden
        t_1 = max(blades[c_b_i-i].max_,t)
        time += (math.log(t_1)-math.log(s))*cb.speed/math.log(1/2)
        s = t_1
       
    print(time)