import heapq

def getAvgWaitTime(n, cust):
    heap = []
    curTime = 0
    waitTime = 0
    cust = sorted(cust)
    i = 0

    while i < n:
        if curTime < cust[i][0]:
            curTime = cust[i][0]
        while i < n and (curTime >= cust[i][0]):
            heapq.heappush(heap, (cust[i][1], cust[i][0]))
            i += 1

        while (i < n) and curTime < cust[i][0] and len(heap) > 0:
            time, wait = calculateWaiting(heap, curTime)
            curTime += time
            waitTime += wait

    # Clear all the jobs
    while len(heap) > 0:
        time, wait = calculateWaiting(heap, curTime)
        curTime += time
        waitTime += wait

    return waitTime / n


def calculateWaiting(heap, curTime):
    wait = 0
    cur = heapq.heappop(heap)
    wait = curTime - cur[1] + cur[0] 
    return (cur[0], wait)


n = int(raw_input().strip())
cust = []
for i in range(n):
    ar = map(int, raw_input().strip().split(' '))
    cust.append((ar[0], ar[1]))


result = getAvgWaitTime(n, cust)
print resultprint(result)