#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>

using namespace std;

#define int long long
#define rep(i,n) for(int i = 0; i < (n); i++)
#define INF ((long long)1e18)
#define MOD ((int)1e9+7)
#define endl "\n"

#define yn(f) ((f)?"Yes":"No")
#define YN(f) ((f)?"YES":"NO")

#define MAX 110000

int N, M;
int a, b;
int hoge[MAX][3];

signed main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout<<fixed<<setprecision(10);
	
	int ans = 0, temp, x = 0, num = 0;
	
	cin>>N>>M;
	
	cin>>a>>b;
	
	x =  max(b,a*(M-1)+max(a,b));
	
	for(int i = 1; i < N; i++){
		cin>>a>>b;
		
		num = max(a,b);
		
		temp = max(b,a*(M-1)+num);
		
		hoge[i+1][2] += temp+hoge[i][2];
		
		hoge[i+1][1] = max(hoge[i][2],hoge[i][1]) +num;
		
		hoge[i+1][0] = max(hoge[i][0],max(hoge[i][2],hoge[i][1]));
	}
	
	ans = max(hoge[N][0],max(hoge[N][2],hoge[N][1]))+x;
	
	cout<<ans<<endl;
	
	return 0;
}