///https://www.instagram.com/_modwwe/
#include<bits/stdc++.h>
#define int long long
//#define ll long long
#define down cout<<'\n';
#define NHP ios_base::sync_with_stdio(0);cout.tie(0);cin.tie(0);
#define modwwe int t;cin>>t; while(t--)
#define bit(i,j) (i>>j&1)
#define sobit(a) __builtin_popcountll(a)
#define task "test"
#define fin(x) freopen(x".inp","r",stdin)
#define fou(x) freopen(x".out","w",stdout)
#define pb push_back
#define checktime cerr << (double)clock() / CLOCKS_PER_SEC * 1000 << " ms";
using namespace std;
void phongbeo();
const int inf=1e18;
const int mod2=1e9+7;
const int mod1=998244353;
struct icd
{
int a,b;
};
struct ib
{
int a;
int b;
};
struct ic
{
int a,b,c;
};
struct id
{
int a,b,c,d;
};
struct ie
{
int a,b,c, d,e,f;
};
int n,m,s1,s2,s4,s3,sf,k,r,mid,s5,s6,mx,s7,s8,s9,mx2,res,dem2=0,dem=0,l;
int i,s10,s12;
int el=29;
main()
{
#ifndef ONLINE_JUDGE
// fin(task),fou(task);
#endif
NHP
/// cin>>s1;
// modwwe
phongbeo();
}
bool cmp(ib a,ib b)
{
return a.a<b.a;
}
ib a[302];
ib b[601];
int d[2];
int dp[607][307][307];
void phongbeo()
{
cin>>n>>k;
for(int i=0; i<=600; i++)
b[i]= {0,inf};
for(int i=1; i<=n; i++)
cin>>a[i].a>>a[i].b,b[a[i].a].a++,b[a[i].a].b=min(b[a[i].a].b,a[i].b),m=max(m,a[i].a);
for(int i=1;i<=m*2;i++)
b[i].b=min(b[i].b,b[i-1].b);
for(int i=0; i<=m*2; i++)
for(int j=0; j<=n; j++)
for(int z=0; z<=n; z++)
dp[i][j][z]=inf;
dp[0][b[0].a][1]=0;
s3=inf;
for(int i=0; i<=m*2; i++)
for(int j=0; j<=n; j++)
for(int z=0; z<=n; z++)
if(dp[i][j][z]!=inf)
{
s2=max(0ll,j-z);
dp[i+1][s2+b[i+1].a][z]=(dp[i+1][s2+b[i+1].a][z],dp[i][j][z]+s2*k);
if(i!=0&&s2!=0) dp[i][j][z+1]=min(dp[i][j][z+1],dp[i][j][z]+b[i-1].b);
if(i>=m&&s2==0)s3=min(s3,dp[i][j][z]);
}
cout<<s3;
}
