N=int(input())
list1=[0]
list2=[]
count=0
for i in range(N-1):
    cost=int(input())
    list1.append(cost)
#print(list1)
for j in range(N):
    list2.append(count)
    line=input()
    line=line.rstrip().split(" ")
    a=int(line[0])
    b=int(line[1])
    count=count-a+b
    
#print(list2)
cost=0
for k in range(N):
    cost=cost+list1[k]*list2[k]
print(cost)