#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=310, inf=1e18;
int n, z, f[N*2][N][N], c[N*2], cnt[N*2];
pair<int, int> a[N];
vector<int> vals;
int d[N*2];
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> z;
map<int, int> mp;
for (int i=1; i<=n; ++i){
cin >> a[i].first >> a[i].second;
++a[i].first;
++mp[a[i].first];
}
{
int cur=0;
for (auto &i:mp){
cur=max(0ll, cur+i.second-1);
for (int j=0; j<=cur; ++j) vals.push_back(i.first+j);
}
}
vals.push_back(0);
sort(vals.begin(), vals.end()); vals.resize(unique(vals.begin(), vals.end())-vals.begin());
assert((int)vals.size()<N*2);
for (int i=1; i<=n; ++i) a[i].first=lower_bound(vals.begin(), vals.end(), a[i].first)-vals.begin(), ++cnt[a[i].first];
for (int i=0; i<(int)vals.size()-1; ++i) d[i]=vals[i+1]-vals[i];
for (int i=0; i<N*2; ++i) c[i]=inf;
for (int i=1; i<=n; ++i) c[a[i].first+1]=min(c[a[i].first+1], a[i].second);
for (int i=1; i<N*2; ++i) c[i]=min(c[i], c[i-1]);
for (int i=0; i<N*2; ++i) for (int j=0; j<N; ++j) for (int k=0; k<N; ++k) f[i][j][k]=inf;
f[0][1][0]=0;
for (int i=0; i<N*2-1; ++i) for (int j=1; j<=n; ++j) for (int k=0; k<=n; ++k) if (f[i][j][k]!=inf){
for (int x=min(cnt[i+1]+k, j); x<=cnt[i+1]+k; ++x){
if (c[i+1]==inf && x-j>0) continue;
f[i+1][max(j, x)][k+cnt[i+1]-x]=min(f[i+1][max(j, x)][k+cnt[i+1]-x], f[i][j][k]+max(0ll, x-j)*c[i+1]+(k+cnt[i+1]-x)*z*d[i+1]);
}
}
int ans=inf;
for (int i=0; i<=n; ++i) ans=min(ans, f[(int)vals.size()-1][i][0]);
cout << ans << '\n';
return 0;
}
