#include<bits/stdc++.h>
using namespace std;
#define int long long
int n,m,sum[100005],sumd[100005];
int a[100005],b[100005],f[100005],suf[100005];
signed main(){
	// freopen("remake.in","r",stdin);
	// freopen("remake.out","w",stdout);
	cin>>n>>m;
	for(int i=1;i<=n;i++)cin>>a[i]>>b[i];
	int ans=LLONG_MIN;
	for(int i=1;i<=n;i++){
		if(b[i]-a[i]>0)sumd[i]=sumd[i-1]+b[i];
		else sumd[i]=sumd[i-1]+a[i];
	}
	suf[n+1]=INT_MIN;
	for(int i=n;i>=1;i--){
		suf[i]=max(suf[i+1],sumd[i]); 
	} 
	for(int i=1;i<=n;i++){
		f[i]=max(max(b[i],m*a[i]),(m-1)*a[i]+b[i])+f[i-1];
		ans=max(ans,f[i]+max(suf[i+1]-sumd[i],0ll));
        // cout<<max(suf[i+1]-sumd[i],0ll);
	}
    // cout<<f[n]<<endl;
	cout<<ans<<endl;
}