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

ans = 0
pas = C[0]
for i in range(1, N):
    ans += pas*A[i-1]
    pas -= B[i]
    pas += C[i]

print(ans)