#include<iomanip>
#include<limits>
#include<thread>
#include<utility>
#include<iostream>
#include<string>
#include<algorithm>
#include<set>
#include<map>
#include<vector>
#include<stack>
#include<queue>
#include<cmath>
#include<numeric>
#include<cassert>
#include<random>
#include<chrono>
#include<unordered_set>
#include<unordered_map>
#include<fstream>
#include<list>
#include<functional>
#include<bitset>
#include<complex>
#include<tuple>
using namespace std;
typedef unsigned long long int ull;
typedef long long int ll;
typedef pair<ll,ll> pll;
typedef pair<int,int> pi;
typedef pair<double,double> pd;
typedef pair<double,ll> pdl;
#define F first
#define S second
const ll E=1e18+7;
const ll MOD=1000000007;




int main(){
    ll n,m;
    cin>>n>>m;
    vector<pll> A(n);
    ll ans=-E;
    ll sum=0;
    ll meat=0;
    ll egg=0;
    for(auto &I:A){cin>>I.F>>I.S;}
    for(auto &I:A){
        meat+=max(I.F,I.S);
        sum+=max(max(I.F*m-max(I.F,I.S),I.F*(m-1)+I.S-max(I.F,I.S)),I.S-max(I.F,I.S));
        if(sum>egg){egg=sum;}
        ans=max(ans,egg+meat);
    }
    cout<<ans<<endl;
    
    
    return 0;
}