#include <iostream>
#include <algorithm>

using namespace std;
typedef long long ll;
ll dp[100010][2] = {},a[100010],b[100010];
int main(){
	ll i,n,m;
	cin >> n >> m;
	for(i=0;i<n;i++){
		cin >> a[i] >> b[i];
	}
	dp[n-1][0] = max(a[n-1],b[n-1]);
	dp[n-1][1] = max({(m-1)*a[n-1] + b[n-1],m*a[n-1],b[n-1]});
	for(i=n-2;i>=0;i--){
		dp[i][0] = max(dp[i+1][0],0LL) + max(a[i],b[i]);
		dp[i][1] = max({dp[i+1][0],dp[i+1][1],0LL}) + max({(m-1)*a[i] + b[i],m*a[i],b[i]});
	}
	cout << dp[0][1] << endl;
}