import sys
input = sys.stdin.readline

n = int(input())
a = [int(input()) for _ in range(n-1)]
now = []
ans = 0

for i in range(n):
    b, c = map(int, input().split())
    
    for _ in range(b):
        ans += now.pop()
    
    for _ in range(c):
        now.append(0)

    for j in range(len(now)):
        now[j] += a[i]
    
print(ans)