#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pii pair<int,int>
#define f(i,a,b) for(int i=(a);i<=(b);i++)
#define Dl(a) cout << #a << " : " << a << "\n";
#define D(a) cout << #a << " : " << a;
#define Da(a,i,j) cout << #a << " : ";f(idx,i,j){cout << a[idx] <<" ";}
const int N = 1e5+10;
int n,m;
int a[N],b[N];
int dp[N][2];
signed main(){
	//freopen(".in","r",stdin);
	//freopen(".out","w",stdout);
	cin >> n >> m;
	f(i,1,n){
		cin >> a[i] >> b[i];
	}
	dp[1][0] = max({a[1] * m,a[1] * (m-1) + b[1],b[1]});
	f(i,2,n){
		dp[i][0] = dp[i-1][0] + max({a[i] * m,a[i] * (m - 1) + b[i],b[i]});
		dp[i][1] = max(dp[i-1][0],dp[i-1][1]) + max(a[i],b[i]);
	}
	int res = -0x3f3f3f3f;
	f(i,1,n){
		res = max({res,dp[i][0],dp[i][1]});
	}
	cout << res;
	return 0;
}