#include<bits/stdc++.h>
#define eb emplace_back
#define ls i<<1
#define rs i<<1|1
using namespace std;
using ll=long long;
using pii=pair<ll,ll>;
constexpr int N=1e5+10,M=1e5+10,K=1e5+10,L=20;
constexpr ll INF=0x3f3f3f3f3f3f3f3f;
constexpr int inf=0x3f3f3f3f;
constexpr int mod=1e9+7;
constexpr long double pi=acos(-1);
constexpr double eps=1e-11;
ll T,n,a[N],b[N],m,sumn[N],ans=-INF,maxn[N],dp[N];
void solve(){
	cin>>n>>m;
	for(int i=1;i<=n;i++){
		cin>>a[i]>>b[i];
		sumn[i]=sumn[i-1]+max(a[i],b[i]);
	}
	for(int i=1;i<=n;i++) maxn[i]=-inf;
	for(int i=n;i>=1;i--){
		maxn[i]=max(maxn[i+1],sumn[i]);
	}
	for(int i=1;i<=n;i++){
		dp[i]=max({b[i],a[i]*m,a[i]*(m-1)+b[i]})+dp[i-1];
		ans=max({ans,dp[i]+maxn[i+1]-sumn[i],dp[i]});
	}
	cout<<ans;
	return;
}
int main(){
	ios::sync_with_stdio(false);
//	freopen("remake.in","r",stdin);
	cin.tie(nullptr);
//	freopen("remake.out","w",stdout);
	T=1;
//	cin>>T;
	while(T--) solve();
	return 0;
}