#include<bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define int long long
#define vi vector<int>
#define vvi vector<vector<int>>
#define pii pair<int, int>
#define vpii vector<pair<int, int>>
#define vc vector<char>
#define vb vector<bool>
#define mii map<int,int>
#define f0r(i,n) for(int i=0;i<n;i++)
#define FOR(i,k,n) for(int i=k;i<n;i++)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define in(a) int a; cin>>a
#define in2(a,b) int a,b; cin>>a>>b
#define in3(a,b,c) int a,b,c; cin>>a>>b>>c
#define in4(a,b,c,d) int a,b,c,d; cin>>a>>b>>c>>d
#define vin(v,n); vi v(n); f0r(i,n){cin>>v[i];}
#define out(a) cout<<a<<'\n'
#define out2(a,b) cout<<a<<' '<<b<<'\n'
#define out3(a,b,c) cout<<a<<' '<<b<<' '<<c<<'\n'
#define out4(a,b,c,d) cout<<a<<' '<<b<<' '<<c<<' '<<d<<'\n'
#define pout(a) cout<<a.first<<' '<<a.second<<'\n'
#define vout(v) for(auto u : v){cout<<u<<' ';} cout<<endl
#define dout(a) cout<<a<<' '<<#a<<endl
#define dout2(a,b) cout<<a<<' '<<#a<<' '<<b<<' '<<#b<<endl
#define yn(x); if(x){cout<<"YES"<<'\n';}else{cout<<"NO"<<'\n';}
const int leg = 1e9 + 7;
const int mod = 998244353;
using namespace std;
const int mxn = 610;
signed main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
//ifstream cin(".in");
//ofstream cout(".out");
in2(n,k);
vi h(n), c(n);
vi cnt(mxn);
f0r(i,n){
cin>>h[i]>>c[i];
cnt[h[i]]++;
}
int ans = 4e18;
int st = h[0];
vi A(2 * mxn + 5);
FOR(goal, 1, n + 1){
f0r(i, mxn)A[i] = 0;
A[st] = 1;
int cur = cnt[st] - 1;
for(int i = st + 1; i < mxn; i++){
if(cnt[i] >= goal){
cur += cnt[i] - goal; A[i] = goal;
}
else{
int rem = min(goal - cnt[i], cur);
cur -= rem; A[i] = cnt[i] + rem;
}
}
int moves = 0;
f0r(i, mxn){
moves += (A[i] - cnt[i]) * i;
}
ans = min(ans, moves * k + (goal - 1) * c[0]);
}
out(ans);
}
