import sys; input = sys.stdin.buffer.readline
sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	N = int(input())
	A = []
	for i in range(N - 1):
		a = int(input())
		A.append(a)

	ans = 0
	b, c = getlist()
	num = c - b
	for i in range(N - 1):
		ans += A[i] * num
		b, c = getlist()
		num += c - b

	print(ans)



if __name__ == '__main__':
	main()