N = int(input())

a = [0] *(N-1)
for i in range(N-1):
    a[i] = int(input())

b = [0] * N
for i in range(N):
    b[i] = list(map(int, input().split()))

ans = 0
p = 0
for i in range(N-1):
    p += b[i][1]
    ans += p * a[i]
    p -= b[i+1][0]

print(ans)