#include<bits/stdc++.h>
#define poly vector<int>
#define IOS ios::sync_with_stdio(false)
#define ll long long
#define mp make_pair
#define mt make_tuple
#define pa pair < int,int >
#define fi first
#define se second
#define inf 1e18
#define mod 998244353
#define sz(x) (int)((x).size())
#define int ll
//#define N
using namespace std;
const int N=305;
int n,K,h[N],c[N];
map<int,int>Mp,all;
int dp[N][N],tmp[N][N],p[N];
inline int cmp(int x,int y)
{
	if (!x||!y) return x+y;
	return (c[p[x]]<c[p[y]]?x:y);
}
void BellaKira()
{
	cin>>n>>K;
	for (int i=1;i<=n;i++)
	{
		cin>>h[i]>>c[i];
		p[i]=i;
	}
	sort(p+1,p+n+1,[&](int x,int y)
	{
		if (h[x]==h[y]) return c[x]<c[y];
		return h[x]<h[y];
	});
	poly g;
	for (int i=n;i>=1;i--) 
	{
		Mp[h[p[i]]]=c[p[i]];
		all[h[i]]++;
		g.push_back(h[i]);
		g.push_back(h[i]+1);
		g.push_back(h[i]+2);
	}
	sort(g.begin(),g.end());
	g.erase(unique(g.begin(),g.end()),g.end());
	memset(dp,0x3f,sizeof(dp));
	memset(tmp,0x3f,sizeof(tmp));
	
	int ans=dp[0][0];
	int tot=all[g[0]],mn=Mp[g[0]];
	dp[1][all[g[0]]-1]=0;
	
	for (int t=1;t<sz(g);t++)
	{
		memset(tmp,0x3f,sizeof(tmp));
		int nw=all[g[t]];
		for (int x=0;x<=tot;x++)
			for (int y=0;x+y<=tot;y++)
			{
				int ny=y+nw;
				if (ny>=x)
					tmp[x][ny-x]=min(tmp[x][ny-x],dp[x][y]+y*K);
				else tmp[x][0]=min(tmp[x][0],dp[x][y]+y*K);
			}
		for (int x=1;x<=tot+nw;x++)
			for (int y=0;y<=tot+nw;y++)
			{
				tmp[x][y]=min(tmp[x][y],tmp[x-1][y+1]+mn);
			}
		memcpy(dp,tmp,sizeof(dp));
		tot+=nw;
		if (nw) mn=min(mn,Mp[g[t]]);
		
		memset(tmp,0x3f,sizeof(tmp));
		int lim=1e14;
		if (t+1<sz(g)) lim=g[t+1]-g[t]-1;
		for (int x=0;x<=tot;x++)
			for (int y=0;y<=tot;y++)
				if (dp[x][y]<ans)
				{
					if (x==0)
					{
						if (lim!=1e14)
							tmp[x][y]=min(tmp[x][y],dp[x][y]+lim*K*y);
					} else
					{
						if (y-lim*x>=0)
						{
							tmp[x][y-lim*x]=min(tmp[x][y-lim*x],
							dp[x][y]+(y-lim*x+x+y)*lim/2*K);
						} else
						{
							tmp[x][0]=min(tmp[x][0],
							dp[x][y]+(y%x+y)*(y/x+1)/2*K);
						}
					}
				}
		memcpy(dp,tmp,sizeof(dp));
	}
	for (int i=1;i<=n;i++)
		ans=min(ans,dp[i][0]);
	cout<<ans<<'\n';
}
signed main()
{
	IOS;
	cin.tie(0);
	int T=1;
	while (T--)
	{
		BellaKira();
	}
}