#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N = 1e5+5;
const ll inf = -1145141919810;
int n,m;
ll a[N],b[N],maxn = inf,tmp,ans,cnt;
bool ok[N];
int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	//freopen("remake.in","r",stdin);
	//freopen("remake.out","w",stdout);
	cin>>n>>m;
	tmp = 0;maxn = inf;
	for(int i=1;i<=n;i++){
		cin>>a[i]>>b[i];
		if(a[i]>0) tmp+=(m-1)*a[i];
		else if(a[i]<0&&b[i]>=a[i]*(m-1)){
			tmp+=b[i];
			ok[i] = true;
		}
		else if(a[i]<0&&b[i]<a[i]*(m-1)) tmp+=(m-1)*a[i];
		if(maxn<tmp){
			cnt = i;
			maxn = tmp;
		}
//		cout<<tmp<<"???"<<ans<<'\n';
	}
	ans = maxn;
	for(int i=1;i<=n;i++){
		if(ok[i]&&i<=cnt) continue;
		ans+=max(a[i],b[i]);
	}
	cout<<ans;
	return 0;
}