#include<bits/stdc++.h>
using namespace std;
#define int int64_t
#define pb push_back
const int lim=510;
int dp[lim][lim]{};
signed main(){
int n,k;
cin>>n>>k;
vector<int>process;
map<int,int>cnt,mn;
for(int i=0;i<n;i++){
int x,y;
cin>>x>>y;
cnt[x]++;
if(!mn.count(x)){
mn[x]=y;
}else{
mn[x]=min(mn[x],y);
}
process.pb(x);
process.pb(x+1);
}
process.pb(INT_MAX);
sort(process.begin(),process.end());
process.resize(unique(process.begin(),process.end())-process.begin());
int m=process.size();
int prefbest[m];
prefbest[0]=LLONG_MAX;
for(int i=1;i<m;i++){
prefbest[i]=prefbest[i-1];
if(mn.count(process[i-1])){
prefbest[i]=min(prefbest[i-1],mn[process[i-1]]);
}
}
for(int i=0;i<=n;i++){
for(int j=0;j<=n;j++){
dp[i][j]=LLONG_MAX;
}
}
dp[1][cnt[process[0]]-1]=0;
auto print=[&](){
for(int i=0;i<=n;i++){
for(int j=0;j<=n;j++){
cerr<<(dp[i][j]==LLONG_MAX?-1:dp[i][j])<<' ';
}cerr<<'\n';
}cerr<<'\n';
};
for(int i=1;i<m;i++){
int x=i&1;
// print();
for(int save=1;save<=n;save++){
for(int tolift=1;((tolift+save-1)/save)<process[i]-process[i-1];tolift++){
int ch=0;
for(int have=1;have<n;have++){
if(dp[save][have]==LLONG_MAX)continue;
if(dp[save][have]+((tolift+save-1)/save)*k<dp[save][have-1]){
dp[save][have-1]=dp[save][have]+((tolift+save-1)/save)*k;
ch=1;
}
}
if(!ch)break;
}
}
// print();
if(process[i]-process[i-1]!=1){
int ch;
do{
ch=0;
for(int save=0;save<n;save++){
for(int have=1;have<=n;have++){
if(dp[save][have]==LLONG_MAX)continue;
if(dp[save][have]+k+prefbest[i]<dp[save+1][have-1]){
dp[save+1][have-1]=dp[save][have]+k+prefbest[i];
ch=1;
}
}
}
}while(ch);
}
// print();
int cc=cnt[process[i]];
for(int save=0;save<=n;save++){
for(int have=n;n-cc<have;have--){
dp[save][have]=LLONG_MAX;
}
for(int have=n-cc;0<=have;have--){
if(dp[save][have]==LLONG_MAX)continue;
dp[save][have+cc]=dp[save][have]+k*(process[i]-process[i-1])*have;
if(cc)dp[save][have]=LLONG_MAX;
}
}
// print();
for(int save=0;save<=n;save++){
for(int have=1;have<=n;have++){
if(dp[save][have]==LLONG_MAX)continue;
dp[save][max<int>(0,have-save)]=min(dp[save][have],dp[save][max<int>(0,have-save)]);
dp[save][have]=LLONG_MAX;
}
}
// print();
for(int have=n;have;have--){
for(int save=0;save<n;save++){
if(dp[save][have]==LLONG_MAX)continue;
if(dp[save][have]+prefbest[i]<dp[save+1][have-1]){
dp[save+1][have-1]=dp[save][have]+prefbest[i];
}
}
}
// print();
// cerr<<"NXT\n\n\n";
}
int ans=LLONG_MAX;
for(int i=0;i<=n;i++){
ans=min(ans,dp[i][0]);
}
cout<<ans<<'\n';
}
