import sequtils,strutils,algorithm,math,sugar,macros,strformat
import sets,tables,intsets,queues,heapqueue,bitops
template get*():string = stdin.readLine().strip()
macro unpack*(arr: auto,cnt: static[int]): auto =
  let t = genSym(); result = quote do:(let `t` = `arr`;())
  for i in 0..<cnt: result[1].add(quote do:`t`[`i`])

let n = get().parseInt()
let A = newSeqWith(n-1,get().parseInt())
var ans = 0
var x = 0
for i in 0..<n:
  let (b,c) = get().split.map(parseInt).unpack(2)
  if i > 0 : ans += x * A[i-1]
  x += c - b
echo ans