#include <bits/stdc++.h>
/*
If all H_i are different, then 0 cost
ST2: must root at 1, everything built at 1
If min, must increase
ST3: permutation
If cost and height are both less, then do not rearrange them, as less cost available sooner
Height same as before -> need extra "connection facility"
Increase height of maximum cost only
Number of extra connections req = max num with same height - 1
O(n^3) dp with this
*/
int main(){
int n,k;
std::cin>>n>>k;
std::vector<std::pair<long long,long long>> vec;
for(int i=0;i<n;i++){
int a,b;
std::cin>>a>>b;
vec.push_back({a,b});
if(a>300)return 0;//no
}
std::sort(vec.begin(),vec.end());
long long basecost=0;
for(int i=1;i<n;i++){
if(vec[i].first==vec[0].first){
vec[i].first++;
basecost+=k;
}
}
std::vector<int> islots(301);
std::vector<std::pair<long long,long long>> places;//where a new min is
int curmin=1e9+1;
for(int i=0;i<n;i++){
islots[vec[i].first]++;
if(vec[i].second<curmin){
curmin=vec[i].second;
places.push_back(vec[i]);
}
}
int ind=0;
long long mincost=1e18;
std::vector<std::vector<long long>> vals;
for(int i=1;i<=n;i++){
std::vector<int> slots(600);
for(int j=0;j<=300;j++){
slots[j]=islots[j];
}
long long ecost=0;
int extra=0;
for(int j=places[ind].first+1;j<600;j++){
extra+=slots[j];
if(extra>i){
slots[j]=i;
ecost+=extra-i;
extra-=i;
}else{
slots[j]=extra;
extra=0;
}
}
long long fcost=ecost*k+(i-1)*places[ind].second;
mincost=std::min(mincost,fcost);
/*
int eind=places[ind].first+1;//actual extra
for(int j=0;j<300;j++){
long long fcost=ecost*k+(i-1)*places[ind].second;
mincost=std::min(mincost,fcost);
while(slots[eind]>=i){
eind++;
}
slots[eind]++;
ecost+=eind-(places[ind].first+1);
}
*/
}
std::cout<<mincost+basecost<<'\n';
}
