1. import heapq
2.  
3. def getAvgWaitTime(n, cust):
4.     heap = []
5.     curTime = 0
6.     waitTime = 0
7.     cust = sorted(cust)
8.     i = 0
9.  
10.     while i < n:
11.         if curTime < cust[i][0]:
12.             curTime = cust[i][0]
13.         while i < n and (curTime >= cust[i][0]):
14.             heapq.heappush(heap, (cust[i][1], cust[i][0]))
15.             i += 1
16.  
17.         while (i < n) and curTime < cust[i][0] and len(heap) > 0:
18.             time, wait = calculateWaiting(heap, curTime)
19.             curTime += time
20.             waitTime += wait
21.  
22.     # Clear all the jobs
23.     while len(heap) > 0:
24.         time, wait = calculateWaiting(heap, curTime)
25.         curTime += time
26.         waitTime += wait
27.  
28.     return waitTime / n
29.  
30.  
31. def calculateWaiting(heap, curTime):
32.     wait = 0
33.     cur = heapq.heappop(heap)
34.     wait = curTime - cur[1] + cur[0]
35.     return (cur[0], wait)
36.  
37.  
38. n = int(raw_input().strip())
39. cust = []
40. for i in range(n):
41.     ar = map(int, raw_input().strip().split(' '))
42.     cust.append((ar[0], ar[1]))
43.  
44.  
45. result = getAvgWaitTime(n, cust)
46. print(result)