n = int(input())
a = [0]*n
for i in range(1,n):
	a[i] = int(input())
b = [0]*n
c = [0]*n

for i in range(n):
	b[i],c[i] = map(int,input().split())

ans = 0
cnt = 0
for i in range(n):
	ans += cnt * a[i]
	cnt -= b[i]
	cnt += c[i]
print(ans)