#include<bits/stdc++.h>
using namespace std;

const int N = 1e5;
long long n, m, a[N + 5], b[N + 5];
long long ans, dp[N + 5][2];

int main(){
//	freopen("remake_sample2.in", "r", stdin);
//	freopen("remake.out", "w", stdout);
	cin >> n >> m;
	for(int i = 1; i <= n; i++)
		cin >> a[i] >> b[i];
	for(int i = 1; i <= n; i++){
		dp[i][0] = dp[i - 1][0] + max(b[i], max(m * a[i], (m - 1) * a[i] + 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;
}