N = int(input())
A = [int(input()) for _ in range(N - 1)]
B, C = [0] * N, [0] * N
for i in range(N):
    B[i], C[i] = map(int, input().split())
X = C[0]
ans = 0
for i in range(N - 1):
    ans += X * A[i]
    X = X - B[i + 1] + C[i + 1]
print(ans)