#include<iostream>
#include<stack>
#include<map>
#include<vector>
#include<string>
#include<cassert>
#include<unordered_map>
#include <queue>
#include <cstdint>
#include<cstring>
#include<limits.h>
#include<cmath>
#include<set>
#include<algorithm>
#include <iomanip>
#include<numeric>
#include<bitset>
using namespace std;
#define ll long long
#define f first
#define s second
#define pii pair<int,int>
#define ppii pair<int,pii>
#define vi vector<int>
#define pb push_back
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define F(n) for(int i=0;i<n;i++)
#define lb lower_bound
#define ub upper_bound
#define fastio ios::sync_with_stdio(false);cin.tie(NULL);
#pragma GCC optimize ("03,unroll-lopps")
#define int long long
#define double long double
using namespace std;
const int mod=1e9+7,mxn=303+5,inf=1e18,minf=-1e18,lg=30;
//#undef int
int n,k,m,x,q,y;
void setIO(string name){
ios_base::sync_with_stdio(0); cin.tie(0);
freopen((name+".in").c_str(),"r",stdin);
freopen((name+".out").c_str(),"w",stdout);
}
int H[mxn+10],C[mxn+10],cnt[2*mxn+10];
int cost[2*mxn+10];
int dp[2*mxn+10][mxn+10][mxn+10];
struct seg{
int v[2*mxn+10];
void init(){for(int i=0;i<=2*n+2;i++)v[i]=inf;}
void update(int pos,int val){
pos+=n+1;
v[pos]=val;
for(int i=pos;i>0;i>>=1)v[i>>1]=min(v[i],v[i^1]);
}
int qry(int l,int r){
int ans=inf;
for(l+=n+1,r+=n+1;l<=r;l>>=1,r>>=1){
if(l&1)ans=min(ans,v[l++]);
if(!(r&1))ans=min(ans,v[r--]);
}
return ans;
}
}t;
int32_t main(){
fastio
cin>>n>>k;
int ans=inf;
for(int i=0;i<=2*mxn;i++)cost[i]=inf;
for(int i=1;i<=n;i++){
cin>>H[i]>>C[i];
cnt[H[i]]++;
cost[H[i]]=min(cost[H[i]],C[i]);
}
for(int i=1;i<=2*mxn;i++)cost[i]=min(cost[i-1],cost[i]);
for(int h=0;h<=2*mxn;h++){
for(int have=n;have>=0;have--)for(int head=0;head<=n;head++)dp[h][have][head]=inf;
}
t.init();
dp[0][0][0]=0;
for(int h=0;h<=2*mxn;h++){
int yes=1;
if(cnt[h]){
yes=0;
for(int have=n;have>=0;have--)for(int head=0;head<=n;head++){
//add cnt into have
//adding one cus we fix one of them to be main chain
if(have-cnt[h]+1>=0)dp[h][have][head]=dp[h][have-cnt[h]+1][head];
else dp[h][have][head]=inf;
}
}
for(int head=0;head<=n;head++){
for(int have=0;have<=n;have++){
t.update(have,dp[h][have][head]);
}
for(int have=0;have<=n;have++){
//removing head from have
//adding yes because we can attach to the main chain for free
//we need min from have-have+head+yes
dp[h][have][head]=t.qry(have,min(n,have+head+yes));
/*
if(have+head+yes<=n){
for(int j=have;j<=min(n,have+head+yes);j++)dp[h][have][head]=min(dp[h][have][head],dp[h][j][head]);
}*/
}
}
for(int have=n;have>=0;have--)for(int head=0;head<=n;head++){
//creating new head
if(head&&h&&have+1<=n)dp[h][have][head]=min(dp[h][have][head],dp[h][have+1][head-1]+cost[h-1]);
}
for(int have=n;have>=0;have--)for(int head=0;head<=n;head++){
dp[h+1][have][head]=min(dp[h+1][have][head],dp[h][have][head]+have*k);
}
}
for(int i=0;i<=n;i++)ans=min(ans,dp[2*mxn][0][i]);
cout<<ans;
}
/*
dp[hi][have][head]
606 x 303 x 303
there has to be 1 fix main chain
where for each height the node with min cost will not move?
*/
