n=int(input())
fare=[]
num=[]
for i in range(n-1):
    fare.append(int(input()))
                
for i in range(n):
    num.append(list(map(int,input().split())))



total=0
ans=0
for i in range(n-1):
    total+=(num[i][1]-num[i][0])
    ans+=total*fare[i]
    
print(ans)