#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
typedef long long ll;

ll N, M;
ll A[100000], B[100000];
ll gain[100000][3];
ll dp[100000][3];

const ll INF = 1e18;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    cin >> N >> M;
    for(int i = 0; i < N; i++) cin >> A[i] >> B[i];

    for(int i = 0; i < N; i++){
        gain[i][0] = max({A[i]*M, A[i]*(M-1)+B[i], B[i]});
        gain[i][1] = max(A[i], B[i]);
        gain[i][2] = 0;

        // cout << gain[i][0] << ' ' << gain[i][1] << ' ' << gain[i][2] << endl;
    }

    dp[0][0] = gain[0][0];
    dp[0][1] = B[0];
    dp[0][2] = B[0];

    for(int i = 1; i < N; i++){
        dp[i][0] = dp[i-1][0]+gain[i][0];
        dp[i][1] = max({dp[i-1][0]+gain[i][1], dp[i-1][1]+gain[i][1]});
        dp[i][2] = max({dp[i-1][0]+gain[i][2], dp[i-1][1]+gain[i][2], dp[i-1][2]+gain[i][2]});
    }
    cout << max({dp[N-1][0], dp[N-1][1], dp[N-1][2]}) << endl;
}