import sys
input = sys.stdin.readline

n=int(input())
A=[int(input()) for i in range(n-1)]

X=[list(map(int,input().split())) for i in range(n)]

now=X[0][1]
ANS=0

for i in range(n-1):
    ANS+=A[i]*now

    now-=X[i+1][0]
    now+=X[i+1][1]

print(ANS)