import math
import sys
def S(): return sys.stdin.readline().rstrip()
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int, sys.stdin.readline().rstrip().split())
def LI(): return list(map(int, sys.stdin.readline().rstrip().split()))
def LS(): return list(sys.stdin.readline().rstrip().split())

n = I()
a = []
for _ in range(n-1):
    a.append(I())

cnt = 0
ans = 0

for i in range(n-1):
    b, c = MI()
    cnt += c-b
    ans += a[i]*cnt
b, c = MI()

print(ans)