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