//#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,ans,vis[N],ans2; 
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];
	}
	for(int i = 1;i <= n;i ++){
		if(a[i]<0 && (b[i]>=0 || (m-1)*a[i]<b[i])){
			sum+=b[i],vis[i]=1;
		}else{
			sum+=(m-1)*a[i];
		}
		ans=max(ans,sum);
	}
	sum=0;
	for(int i = 1;i <= n;i ++){
		if(!vis[i])	sum+=max(a[i],b[i]);
		ans2=max(ans2,sum);	
	}
	cout << ans+ans2;
	return 0;
}