n = int(input())

a = [0]


num = 0
ans = 0 

for i in range(1,n):
    a.append(int(input()))
    
a.append(0)

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

print(ans)