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