import sys,collections;sys.setrecursionlimit(10**7)
from operator import itemgetter
def Is(): return [int(x) for x in sys.stdin.readline().split()]
def Ss(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline())
def S(): return input()

n = I()
a = [0]
for _ in range(n-1):
    a.append(I())
p = 0
f = 0
for i in range(n):
    b,c = Is()
    f += p*a[i]
    p += c - b
print(f)