//#pragma GCC optimize(3)
#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(){
//	freopen("remake.in","r",stdin);
//	freopen("remake.out","w",stdout);
	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 ++){
		if(b[i]>a[i]){
			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;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;
}