#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <stdio.h>
using namespace std;
#define int long long
int MOD = 1000000007;
signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int N, M;
	cin >> N >> M;
	vector<int> A(N);
	vector<int> B(N);
	int res = 0;
	for (int i = 0; i < N; i++) {
		cin >> B[i] >> A[i];
	}
	vector<int> X(N + 1, 0);
	for (int i = N - 1; i >= 0; i--) {
		int t = max(A[i], B[i]);
		X[i] = max((int)0, X[i + 1] + t);
	}
	int a = 0;
	for (int i = 0; i < N; i++) {
		
		a += max(A[i], max(A[i] + (M - 1)* B[i], M*B[i]));
		//cerr << a + X[i + 1] << endl;
		res = max(res, a + X[i + 1]);
	}
	cout << res << endl;
}