n,d=gets.split.map(&:to_i)
earns=n.times.map{gets.split.map(&:to_i)}

dp=[0,-d]
0.upto(n-1){|i|
  t,k=earns[i]
  _dp=[[dp[0]+t,dp[1]-d+t].max,[dp[0]-d+k,dp[1]+k].max]
  dp=_dp
}

puts dp.max