#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}


//INSERT ABOVE HERE
signed main(){
  Int n,m;
  cin>>n>>m;
  vector<Int> a(n),b(n);
  for(Int i=0;i<n;i++) cin>>a[i]>>b[i];
   
  vector<Int> dp1(n+1,0),dp2(n+1,0);
  for(Int i=1;i<=n;i++)
    dp1[i]=dp1[i-1]+max({b[i-1],a[i-1]*m,a[i-1]*(m-1)+b[i-1]});
  
  for(Int i=n-1;i>=0;i--)
    dp2[i]=max(a[i],b[i])+max<Int>(0,dp2[i+1]);
  
  Int ans=b[0];
  for(Int i=1;i<=n;i++) chmax(ans,dp1[i]+max<Int>(dp2[i],0));  
  cout<<ans<<endl;
  return 0;
}