#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

int main()
{
	int n; ll m; cin>>n>>m;
  ll a[100010], b[100010];
  for(int i=1; i<=n; i++){
    cin>>a[i]>>b[i];
  }
  ll s[100010], s2[100010]; s[0]=0, s2[0]=0;
  for(int i=1; i<=n; i++){
    s[i]=s[i-1]+max({m*a[i], (m-1)*a[i]+b[i], b[i]});
  }
  for(int i=1; i<=n; i++){
    s2[i]=s2[i-1]+max(a[i], b[i]);
  }
  ll mx=-1e18;
  ll ans=-1e18;
  for(int i=n; i>=1; i--){
    mx=max(mx, s2[i]);
    ans=max(ans, mx-s2[i]+s[i]);
  }
  cout<<ans<<endl;
	return 0;
}