#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];

  reverse(a.begin(),a.end());
  reverse(b.begin(),b.end());

  Int c=0,d=0;  
  for(Int i=0;i<n;i++){
    c=max<Int>(c+a[i],0);
    d+=max(a[i],b[i]);
    //cout<<c<<" "<<d<<endl;
  }  
  Int ans=-1e9;
  chmax(ans,d);
  chmax(ans,c*m);
  chmax(ans,c*(m-1)+d); 
  cout<<ans<<endl;
  return 0;
}