#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=1e5+5;
const ll INF=1e16;
int n,m;
ll a[N],b[N];
ll s[N],pre[N],suf[N],ans;
ll f[1005][1005];
void solve1(){
	ans=0;
	for(int i=0;i<=n;i++){
		for(int j=0;j<=m+1;j++) f[i][j]=-INF;
	}
	if(a[1]>=b[1]&&a[1]>=0) f[1][m]=m*a[1];
	else if(a[1]>=b[1]) f[1][m]=max(m*a[1],b[1]);
	else if(a[1]>=0) f[1][m]=(m-1)*a[1]+b[1];
	else f[1][m]=b[1];
	ans=f[1][m];
	for(int i=2;i<=n;i++){
		for(int j=1;j<=m;j++){
			ll maxl=0;
			for(int k=j;k<=m;k++) maxl=max(maxl,f[i-1][k]);
			if(a[i]>=b[i]&&a[i]>=0) f[i][j]=maxl+j*a[i];
			else if(a[i]>=b[i]) f[i][j]=maxl+max(j*a[i],b[i]);
			else if(a[i]>=0) f[i][j]=maxl+(j-1)*a[i]+b[i];
			else f[i][j]=maxl+b[i];
			ans=max(ans,f[i][j]);
		}
	}
	cout<<ans<<"\n";
}
int main(){
	ans=-INF;
	cin>>n>>m;
	for(int i=1;i<=n;i++){
		cin>>a[i]>>b[i];
		if(a[i]>=b[i]) s[i]=s[i-1]+a[i];
		else s[i]=s[i-1]+b[i];
		if(a[i]>=b[i]&&a[i]>=0) pre[i]=pre[i-1]+m*a[i];
		else if(a[i]>=b[i]) pre[i]=pre[i-1]+max(m*a[i],b[i]);
		else if(a[i]>=0) pre[i]=pre[i-1]+(m-1)*a[i]+b[i];
		else pre[i]=pre[i-1]+b[i];
	}
	suf[n]=s[n];
	for(int i=n-1;i>=1;i--) suf[i]=max(suf[i+1],s[i]);
	for(int i=1;i<=n;i++){
		ans=max(ans,max(pre[i],pre[i]+suf[i+1]-s[i]));
	}
	cout<<ans<<"\n";
	return 0;
}