n,m=map(int,input().split())
dp=[[0,0,0] for _ in range(n)]
for i in range(n):
  a,b=map(int,input().split())
  dp[i][0]=max(dp[i-1])
  dp[i][1]=max(dp[i-1][1:])+max(a,b)
  dp[i][2]=dp[i-1][2]+max(m*a,b+(m-1)*a,b)

if n==1:
  print(dp[-1][-1])
else:
  print(max(dp[-1]))