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

typedef long long ll;
const int N = 1e5 + 5;
int n, m;
ll a[N], b[N], f[N][2];

int main(){

	//freopen("remake.in", "r", stdin);
	//freopen("remake.out", "w", stdout);
	ios :: sync_with_stdio(false);
	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], a[i] * m), a[i] * (m - 1) + b[i]);
		f[i][1] = max(f[i - 1][0], f[i - 1][1]) + max(a[i], b[i]);
	}
	ll ans = 0;
	for(int i = 1; i <= n; i++)
		ans = max(ans, max(f[i][0], f[i][1]));
	cout << ans;

	return 0;
}