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