#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=310, inf=1e18;
int n, z, f[N][N][N], c[N], cnt[N];
pair<int, int> a[N];
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> z;
for (int i=1; i<=n; ++i){
cin >> a[i].first >> a[i].second;
++a[i].first;
++cnt[a[i].first];
}
for (int i=0; i<N; ++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; ++i) c[i]=min(c[i], c[i-1]);
for (int i=0; i<N; ++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-1; ++i) for (int j=0; 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);
}
}
int ans=inf;
for (int i=0; i<=n; ++i) ans=min(ans, f[N-1][i][0]);
cout << ans << '\n';
return 0;
}
