#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;
	ll f[N][2];
	for(int i=1;i<=n;i++)
		cin>>a[i]>>b[i];
	f[0][0] = 0;f[0][1] = inf;
	for (int i=1;i<=n;i++){
	    ll maxn = max({b[i],m*a[i],(m-1)*a[i]+b[i]});
	    f[i][0] = f[i-1][0]+maxn;
	    ll take = max(a[i],b[i]);
	    f[i][1] = max({f[i-1][1],f[i-1][0]+take,f[i-1][1]+take});
	    ans = max({ans,f[i][0],f[i][1]});
	}
	cout<<ans;
	return 0;
}