#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define mod 1000000007
using ll=long long;
const int INF=1000000000;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

struct IOSetup{
    IOSetup(){
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout<<fixed<<setprecision(12);
    }
} iosetup;
 
template<typename T1,typename T2>
ostream &operator<<(ostream &os,const pair<T1,T2>&p){
    os<<p.first<<" "<<p.second;
    return os;
}
 
template<typename T>
ostream &operator<<(ostream &os,const vector<T>&v){
    for(int i=0;i<(int)v.size();i++) os<<v[i]<<(i+1==(int)v.size()?"":" ");
    return os;
}

template<typename T1,typename T2>
istream &operator>>(istream &is,pair<T1,T2>&p){
    is>>p.first>>p.second;
    return is;
}

template<typename T>
istream &operator>>(istream &is,vector<T>&v){
    for(T &x:v)is>>x;
    return is;
}


signed main(){
    ll n,m;cin>>n>>m;
    vector<ll> a(n),b(n);
    rep(i,n) cin>>a[i]>>b[i];

    vector<vector<ll>> dp(n+1,vector<ll>(2,-LINF));
    dp[0][0]=0;
    rep(i,n){
        chmax(dp[i+1][0],dp[i][0]+max({b[i],a[i]*(m-1)+b[i],a[i]*m}));
        chmax(dp[i+1][1],dp[i][0]+a[i]);
        chmax(dp[i+1][1],dp[i][0]+b[i]);
        chmax(dp[i+1][1],dp[i][1]+a[i]);
        chmax(dp[i+1][1],dp[i][1]+b[i]);
    }
    ll res=-LINF;
    for(int i=1;i<=n;i++)rep(j,2) chmax(res,dp[i][j]);
    cout<<res<<endl;
    return 0;
}