#include <iostream>
#include <algorithm>
using namespace std;
typedef long long ll;
int main(){
    ll n, m; cin>>n>>m;
    ll a[100001], b[100001];
    ll sa[100001], sa2[100001], sd[100001], mx[100001], sd2[100001];
    sa[0]=sd[0]=sd2[0]=sa2[0]=0; mx[0]=-1e18;
    if(m==1) mx[0]=0;
    for(int i=1; i<=n; i++){
        cin>>a[i]>>b[i];
        if(a[i]>=0){
            sa[i]=sa[i-1]+a[i];
            sa2[i]=sa2[i-1]+a[i];
            sd[i]=sd[i-1]+max(0ll, b[i]-a[i]);
            sd2[i]=sd2[i-1];
        }else{
            sa[i]=sa[i-1]+b[i];
            sa2[i]=sa2[i-1];
            sd[i]=sd[i-1];
            sd2[i]=sd2[i-1]+max(0ll, a[i]-b[i]);
        }
        mx[i]=max(mx[i-1], (m-1)*sa2[i]-sd2[i]);
    }
    ll ans=-1e18;
    for(int i=1; i<=n; i++){
        ans=max(ans, sa[i]+sd[i]+mx[i]+sd2[i]);
    }
    cout<<ans<<endl;
    return 0;
}