#include <bits/stdc++.h>

#define ____AnhKietSS____ ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define NamDinh signed
#define ll long long
#define ii pair<int,int>
#define foru(i,d,c) for(int i=(d);i<=(c);i++)
#define ford(i,d,c) for(int i=(d);i>=(c);i--)
#define fi first
#define se second
#define pb push_back
#define pf push_front
#define MOD 1000000007
#define inf 0x3f
#define INF 4557430888798830399LL

using namespace std;

struct State
{
    ll cost;
    int u;
    int p;
    bool is_free;
};

ll solveSubtaskN40(int n,ll k,vector<ll>&a,vector<ll>&b)
{
    ll z=INF;

    foru(x,1,n)
    {
        vector<bool> c(n+1,false);
        vector<bool> d(n+1,false);
        vector<ll> e(n+1,0);

        c[x]=true;
        d[x]=true;
        e[x]=a[x];

        ll f=0;
        bool g=true;

        foru(i,1,n-1)
        {
            int u=-1;
            int v=-1;
            ll w=INF;
            bool t=false;

            foru(j,1,n)
            {
                if(c[j])
                {
                    continue;
                }

                foru(p,1,n)
                {
                    if(!c[p])
                    {
                        continue;
                    }

                    ll q=k*max(0LL,e[p]+1-a[j]);

                    if(d[p])
                    {
                        if(q<w)
                        {
                            w=q;
                            u=j;
                            v=p;
                            t=true;
                        }
                    }

                    q+=b[p];

                    if(q<w)
                    {
                        w=q;
                        u=j;
                        v=p;
                        t=false;
                    }
                }
            }

            if(u==-1)
            {
                g=false;
                break;
            }

            c[u]=true;
            d[u]=true;
            e[u]=max(a[u],e[v]+1);

            f+=w;

            if(t)
            {
                d[v]=false;
            }
        }

        if(g)
        {
            z=min(z,f);
        }
    }

    return z;
}

NamDinh main()
{
    ____AnhKietSS____



    int n;
    ll k;

    cin>>n>>k;

    vector<ll>a(n+1),b(n+1);

    foru(i,1,n)
    {
        cin>>a[i]>>b[i];
    }

    if(n==1)
    {
        cout<<0<<"\n";
        return 0;
    }

    cout<<solveSubtaskN40(n,k,a,b)<<"\n";

    return 0;
}