#include <stdio.h>
#include <string>
#include <cstring>
#include <stdlib.h>
#include <math.h>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <list>
#include <iterator>
#pragma warning(disable:4996)
 
typedef long long ll;
#define MIN(a, b) ((a)>(b)? (b): (a))
#define MAX(a, b) ((a)<(b)? (b): (a))
#define LINF 9223300000000000000
#define INF 2140000000
#define MOD 1000000007
using namespace std;

ll dp[100005][3];

int main(int argc, char* argv[])
{
    long n, m;
    scanf("%ld%ld", &n, &m);

    vector<long> a(n),b(n);
    long i;
    for(i=0; i<n; i++) {
        scanf("%ld%ld", &a[i], &b[i]);
    }

    dp[n][0]=dp[n][1]=dp[n][2]=0;
    for(i=n-1; i>=0; i--) {
        dp[i][0]=0;
        dp[i][1]=MAX(dp[i+1][0],dp[i+1][1])+MAX(a[i],b[i]);
        dp[i][2]=MAX(dp[i+1][0],MAX(dp[i+1][1],dp[i+1][2]))+MAX((ll)a[i]*m,MAX(b[i]+(ll)a[i]*(m-1),b[i]));
    }
    printf("%lld\n", dp[0][2]);

    return 0;
}