import strutils, sequtils, math

proc main() =
  var
    nm = stdin.readLine.split.map(parseInt)
    (n, m) = (nm[0], nm[1])
    abi = (0..<n).mapIt(stdin.readLine.split.map(parseBiggestInt))
  
  var
    aacc = newSeq[int64]()
    bestAcc = newSeq[int64]()
  aacc.add(0)
  bestAcc.add(0)
  for ab in abi:
    aacc.add(aacc[^1] + ab[0])
    bestAcc.add(bestAcc[^1] + max(ab[0], ab[1]))
  var ans:int64 = 0
  if bestAcc.max > aacc.max:
    ans += bestAcc.max
    m -= 1
  ans += aacc.max * m
  echo ans
  
main()