//#pragma GCC optimize(3)
#include <bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define fi first
#define se second
#define pb push_back
using namespace std;
const int N = 1e5+10;
int n,m,a[N],b[N],f[N][2],ans; 
signed main(){
//	freopen("remake.in","r",stdin);
//	freopen("remake.out","w",stdout);
	ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
	cin >> n >> m;
	for(int i = 1;i <= n;i ++){
		cin >> a[i] >> b[i];
	}
	for(int i = 1;i <= n;i ++){
		f[i][0]=f[i-1][0]+max(max(b[i],m*a[i]),(m-1)*a[i]+b[i]);
		f[i][1]=max(f[i-1][1],f[i-1][0])+max(a[i],b[i]);
		ans=max(f[i][0],f[i][1]);
	}
	cout << ans;
	return 0;
}