#yuki633

n=int(raw_input())
a=[int(raw_input()) for i in xrange(n-1)]
f=[]
for i in xrange(n):
 b,c=map(int,raw_input().split())
 f.append(c-b)
for i in xrange(1,n):
 f[i]+=f[i-1]
s=[]
for i in xrange(n-1):
 s.append(a[i]*f[i])
print sum(s)