#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    ll n, m;
    cin >> n >> m;
    ll a[n], b[n];
    rep(i,n)cin >> a[i] >> b[i];

    ll dp[n] = {};
    rep(i,n){
        if(i > 0) dp[i] = dp[i-1];
        if(0 <= a[i] && a[i] <= b[i]){
            dp[i] += (m-1)*a[i] + b[i];
        }else if(0 <= b[i] && b[i] <= a[i]){
            dp[i] += m*a[i];
        }else if(a[i] <= 0 && 0 <= b[i]){
            dp[i] += b[i];
        }else if(b[i] <= 0 && 0 <= a[i]){
            dp[i] += m*a[i];
        }else if(a[i] <= b[i] && b[i] <= 0){
            dp[i] += b[i];
        }else if(b[i] <= a[i] && a[i] <= 0){
            dp[i] += max(m*a[i], b[i]);
        }
    }
    ll ans = -1001001001;
    rep(i,n)ans = max(ans, dp[i]);

    ll dp2[n] = {};
    rep(i,n){
        if(i > 0) dp2[i] = dp2[i-1];
        dp2[i] += max(a[i], b[i]);
    }
    ll dp3[n] = {};
    dp3[n-1] = dp2[n-1];
    drep(i,n-1){
        dp3[i] = max(dp2[i], dp3[i+1]);
    }

    rep(i,n-1){
        ll tmp = dp[i] + dp3[i+1] - dp2[i];
        ans = max(ans, tmp);
    }

    cout << ans << endl;
    return 0;
}