from itertools import accumulate
from collections import defaultdict

def main():
    n = int(input())
    a = [0] + [int(input()) for _ in range(n - 1)]
    sum_a = list(accumulate(a))
    d = defaultdict(int)
    ans = 0
    for i in range(n):
        b, c = map(int, input().split())
        for j, k in d.items():
            tmp = sum_a[i] - sum_a[j]
            if k == 0:
                continue
            if b > k:
                ans += tmp * k
                d[j] = 0
                b -= k
            else:
                ans += tmp * b
                d[j] -= b
                break
        d[i] += c

    print(ans)






if __name__ == "__main__":
    main()