n = int(input())
a = [0]
for _ in range(n - 1):
    a.append(int(input()))
for i in range(1, n):
    a[i] += a[i - 1]
ans = 0
for i in range(n):
    b, c = map(int, input().split())
    ans += a[i] * (b - c)
print(ans)