#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
    ll n,m;cin>>n>>m;
    ll a[n+1],b[n+1];for(ll i=1;i<=n;i++)cin>>a[i]>>b[i];
    a[0]=0;b[0]=0;
    ll dp[n+1][2];
    for(ll i=0;i<n+1;i++){
        dp[i][0]=-1000000000000000000;
        dp[i][1]=-1000000000000000000;
    }
    dp[0][0]=0;dp[0][1]=0;
    ll ans;
    if(a[1]<0&&b[1]<0){
      ans=max(b[1],a[1]*m);
    }
  else{
    if(b[1]>=0){
      ans=max(a[1],b[1])+max(a[1]*(m-1),(ll)0);
    }
    else{
      ans=a[1]*m;
    }
  }
    for(ll i=1;i<=n;i++){
        if(a[i]<0&&b[i]<0){
            dp[i][1]=max(dp[i][1],max(dp[i-1][0],dp[i-1][1])+max(a[i],b[i]));
            dp[i][0]=max(dp[i][0],dp[i-1][0]+max(b[i],a[i]*m));
        }
        else{
            dp[i][1]=max(dp[i][1],max(dp[i-1][0],dp[i-1][1])+max(a[i],b[i]));
            if(b[i]>=0){
                dp[i][0]=max(dp[i][0],dp[i-1][0]+max(a[i],b[i])+max(a[i]*(m-1),(ll)0));
            }
            else{
                dp[i][0]=max(dp[i][0],dp[i-1][0]+a[i]*m);
            }
        }
        ans=max(ans,max(dp[i][0],dp[i][1]));
    }
    cout<<ans<<endl;
}