n = int(input())

fee = []
for i in range(n-1):
	fee.append(int(input()))

imos = [0] * (n-1)

for i in range(n-1):
	out,get = map(int,raw_input().split())
	imos[i] += get
	imos[i] -= out

raw_input()

for i in range(1,n-1):
	imos[i] = imos[i-1] + imos[i]

ans = 0
for i in range(n-1):
	ans += fee[i] * imos[i]

print ans