#include <bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define fi first
#define se second
#define pb push_back
using namespace std;
const int N = 1e5+10;
int n,m,a[N],b[N],f[N],sum[N],sumd[N],suf[N],ans; 
signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin>>n>>m;
	for(int i=1;i<=n;i++){
		cin>>a[i]>>b[i];
	}
	ans=INT_MIN;
	for(int i = 1;i <= n;i ++){
		sumd[i]=sumd[i-1]+max(a[i],b[i]);
	}
	suf[n+1]=INT_MIN;
	for(int i = n;i;i --)	suf[i]=max(suf[i+1],sumd[i]);
	for(int i = 1;i <= n;i ++){
		f[i]=max(max(b[i],a[i]*m),(m-1)*a[i]+b[i])+f[i-1];
		ans=max(ans,f[i]+max(suf[i+1]-sumd[i],0ll));
	}
	cout << ans;
	return 0;
}