n = int(input())
al = list()
bcl = list()
for i in range(n-1):
    al.append(int(input()))
for i in range(n):
    bcl.append([int(x) for x in input().split()])

ans = 0
pas = 0
for i in range(n-1):
    bc = bcl[i]
    pas -= bc[0]
    pas += bc[1]
    ans += pas * al[i]

print(ans)