n = int(input())
payment = [ int(input()) for i in range(n-1)]
passenger = [ [ int(v) for v in input().split() ] for i in range(n) ]
passenger = [ (passenger[i][1] - passenger[i][0]) for i in range(n) ]
bus = [ sum(passenger[:i+1]) for i in range(n-1) ]
total = sum([(payment[i]*bus[i]) for i in range(n-1)])

print(total)