#include<bits/stdc++.h>
using namespace std;
#define int long long
#define F first
#define S second
#define all(x) x.begin(),x.end()
#define pii pair<int,int>
#define pb push_back
#define sz(x) (int)(x.size())
#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)
#define vi vector<int>
#define vp vector<pii>
#define vvi vector<vi>
#define ykh mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count())
#define __lg(x) 63-__builtin_clzll(x)
#define pow2(x) (1LL<<x)
void __print(int x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef local
void CHECK();
void setio(){
freopen("/Users/iantsai/cpp/input.txt","r",stdin);
freopen("/Users/iantsai/cpp/output.txt","w",stdout);
}
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
void setio(){}
#define debug(x...)
#endif
#define TOI_is_so_de ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);setio();
const int inf = 1e18;
void solve(){
int n, K;
cin >> n >> K;
vector<int>h(n + 1), c(n + 1), tmp;
for(int i = 1; i <= n; i++){
cin >> h[i] >> c[i];
tmp.pb(h[i]);
tmp.pb(h[i] + 1);
}
sort(all(tmp));
tmp.resize(unique(all(tmp)) - tmp.begin());
vector<int>cnt(sz(tmp)), mn(sz(tmp), inf);
for(int i = 1; i <= n; i++){
int p = lower_bound(all(tmp), h[i]) - tmp.begin();
chmin(mn[p], c[i]);
cnt[p]++;
}
debug(mn, cnt);
int dp[sz(tmp) + 2][n + 2][n + 1];
for(int i = 0; i <= sz(tmp); i++){
for(int j = 0; j <= n + 1; j++){
for(int k = 0; k <= n; k++){
dp[i][j][k] = inf;
}
}
}
dp[0][1][0] = 0;
auto F = [&](int n, int m, int k){
chmin(n, k / m);
return n * (n - 1) / 2 * m + (k - n * m) * n;
};
for(int i = 0; i < sz(tmp); i++){
if(i) chmin(mn[i], mn[i - 1]);
int wid = (i == sz(tmp) - 1 ? n : min(n, tmp[i + 1] - tmp[i]));
for(int j = 1; j <= n; j++){
for(int k = 0; k <= n; k++){
if(dp[i][j][k] == inf) continue;
if(i) chmin(dp[i][j + 1][k], dp[i][j][k] + mn[i - 1]);
debug(i, j, k, dp[i][j][k]);
chmin(dp[i + 1][j][max(0LL, cnt[i] + k - j * wid)], dp[i][j][k] + K * F(wid, j, k + cnt[i]));
}
}
}
int ans = inf;
for(int j = 1; j <= n; j++){
chmin(ans, dp[sz(tmp)][j][0]);
}
cout << ans << '\n';
}
signed main(){
TOI_is_so_de;
int t = 1;
//cin >> t;
while(t--){
solve();
}
#ifdef local
CHECK();
#endif
}
/*
input:
*/
#ifdef local
void CHECK(){
cerr << "\n[Time]: " << 1000.0 * clock() / CLOCKS_PER_SEC << " ms.\n";
function<bool(string,string)> compareFiles = [](string p1, string p2)->bool {
std::ifstream file1(p1);
std::ifstream file2(p2);
if(!file1.is_open() || !file2.is_open()) return false;
std::string line1, line2;
while (getline(file1, line1) && getline(file2, line2)) {
if (line1 != line2)return false;
}
int cnta = 0, cntb = 0;
while(getline(file1,line1))cnta++;
while(getline(file2,line2))cntb++;
return cntb - cnta <= 1;
};
bool check = compareFiles("output.txt","expected.txt");
if(check) cerr<<"ACCEPTED\n";
else cerr<<"WRONG ANSWER!\n";
}
#else
#endif
