#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];
  
  Int ans=-1e9;
  Int res=0,dif=0,val=0;
  for(Int i=0;i<n;i++){
    res+=max(a[i],b[i]);
    dif+=a[i];
    chmax(val,dif);
    chmax(ans,res+val*(m-1));    
  }
  
  cout<<ans<<endl;
  return 0;
}