n = int(input())
a = []
ppl = 0
ans = 0

for x in range(0,n-1):
    a.append(input())

b, c = map(int,input().split())
ppl += c

print(a)
for y in a:
    b, c = map(int,input().split())
    ans = (ppl * int(y)) + ans
    ppl = ppl - b + c

print(ans)