import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N = int(readline())
A = tuple(int(readline()) for _ in range(N-1))
m = map(int,read().split())
B,C = zip(*zip(m,m))

A

B,C

x = 0
answer = 0
for a,b,c in zip(A,B[1:],C[:-1]):
    x += c
    answer += a * x
    x -= b

print(answer)