///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();
}
int dp[607][307][307];
ib a[307];
ib b[707];
vector<int> v;
int get2(int l)
{
return (l+1)*l/2;
}
int get(int l,int r)
{
l-=r;
if(l<=0) return 0;
else
{
int ss2=l/r;
int ss3=l%r;
return get2(ss2)*r+(ss2+1)*ss3;
}
}
void phongbeo()
{
cin>>n>>k;
for(int i=1; i<=n; i++)
cin>>a[i].a>>a[i].b,v.pb(a[i].a),v.pb(a[i].a+1),s6=max(s6,a[i].a);
sort(v.begin(),v.end());
v.erase(unique(v.begin(),v.end()),v.end());
v.pb(v.back()+n-1);
for(int i=0; i<=700; i++)
b[i].b=inf;
for(int i=1; i<=n; i++)
a[i].a=lower_bound(v.begin(),v.end(),a[i].a)-v.begin(),
b[a[i].a].a++,
b[a[i].a].b=min(b[a[i].a].b,a[i].b);
m=v.size()-1;
for(int i=1; i<=m; i++)
b[i].b=min(b[i].b,b[i-1].b);
for(int i=0; i<=m; 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;
s7=inf;
for(int i=0; i<=m; i++)
for(int j=b[i].a; j<=n; j++)
for(int z=0; z<=n; z++)
if(dp[i][j][z]!=inf)
{
if(i!=0&&j>z) dp[i][j][z+1]=min(dp[i][j][z+1],dp[i][j][z]+b[i-1].b);
if(i!=m)
{
s3=v[i+1]-v[i];
s2=s3*z;
if(s2>=j)
{
dp[i+1][b[i+1].a][z]=min(dp[i+1][b[i+1].a][z],dp[i][j][z]+get(j,z)*k);
}
else
{
s2=s3*z;
s4=j-s2;
dp[i+1][b[i+1].a+s4][z]=min(dp[i+1][b[i+1].a+s4][z],dp[i][j][z]+(get(s2,z)+s4*s3)*k);
}
}
if(v[i]>=s6&&j<=z)
{
s7=min(s7,dp[i][j][z]);
}
}
cout<<s7;
}
