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