n,m=map(int,input().split())
dp=[[0,0,0] for _ in range(n)]
a,b=map(int, input().split())
dp[0][0]=dp[0][1]=-10**18
dp[0][2]=max(m*a,b+(m-1)*a,b)
for i in range(1, 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)


print(max(dp[-1]))