#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1e5+5;
ll n,m,ans=0;
ll a[N],b[N],f[N];
bool vis[N];
vector<ll> t1,t2;
int main(){
	scanf("%lld%lld",&n,&m);
	for(ll i=1;i<=n;i++)	scanf("%lld%lld",a+i,b+i);
	ll mx=0,sum=0;
	for(ll i=1;i<=n;i++){
		f[i]=f[i-1]+a[i]*(m-1);
		if(a[i]<0&&f[i]<f[i-1]+b[i]){
			f[i]=f[i-1]+b[i];
			t1.push_back(i);
		}
		if(f[i]>mx){
			mx=f[i];
			for(int j=0;j<t1.size();j++)	t2.push_back(t1[j]);	
			t1.clear();
		}
	}
	ans+=mx;
	for(ll i=0;i<t2.size();i++)	vis[t2[i]]=true;
	mx=0;
	for(ll i=1;i<=n;i++){
		if(vis[i])	continue;
		sum+=max(a[i],b[i]);
		mx=max(sum,mx);
	}
	ans+=mx;
	cout<<ans;
	return 0;
}