#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
typedef long long ll;
const int N=1e5+10;
ll n,m;
ll a[N],b[N];
ll dp[N][2];
ll ans=-1e18;
int main()
{
	cin>>n>>m;
	for(int i=1;i<=n;i++)
		cin>>a[i]>>b[i];
    memset(dp,0x80,sizeof(dp));
	dp[1][0]=max(b[1],max(a[1]*(m-1)+b[1],a[1]*m));
    for(int i=2;i<=n;i++)
    {
        dp[i][0]=dp[i-1][0]+max(max(a[i]*m,b[i]),a[i]*(m-1)+b[i]);
        dp[i][1]=max(dp[i-1][0],dp[i-1][1])+max(a[i],b[i]);
    }
    for(int i=1;i<=n;i++)
      ans=max(ans,max(dp[i][0],dp[i][1]));
  cout<<ans;
	return 0;
}