#!/usr/bin/env python3
import sys

def main():
    N = int(input())
    A = [int(input()) for _ in range(N - 1)]
    num = 0
    ans = 0
    for i in range(N):
        B, C = map(int,input().split())
        num += C - B
        if i == N - 1:
            break
        ans += A[i] * num
    print(ans)
    return
   
        
if __name__ == '__main__':
    main()