/* Author : Mychecksdead */
#include<bits/stdc++.h>
using namespace std;
#define ll long long int
#define MOD (1000000000+7)
#define MOD1 (998244353)
#define pb push_back
#define all(x) x.begin(), x.end()
#define en cout << '\n'
#define ff first
#define ss second
#define pii pair<int,int>
#define vi vector<int>
const int N = 300+5, M = 1e5+10, K = 52, MX = 30;
const ll INF = 1e14;
int n;
ll k, p[605], dp[605][N][N][2], mn[605], last[N][N], pref[N][N];
array<ll, 2> a[N];
void solve(){
cin >> n >> k;
int m;
for(int i = 1; i <= n; ++i){
cin >> a[i][0] >> a[i][1];
++a[i][0];
m = max(m, (int)a[i][0] + 2);
}
ll ans = INF;
for(int i = 0; i <= m; ++i) p[i] = 0, mn[i] = INF;
for(int i = 1; i <= n; ++i){
p[a[i][0]]++;
mn[a[i][0]] = min(mn[a[i][0]], a[i][1]);
}
for(int i = 1; i <= m; ++i) p[i] += p[i - 1], mn[i] = min(mn[i], mn[i - 1]);
for(int h = 0; h <= m; ++h){
for(int i = 0; i <= n; ++i){
for(int l = 0; l <= n; ++l) dp[h][i][l][0] = dp[h][i][l][1] = INF;
}
}
sort(a+1, a+1+n);
dp[0][0][0][0] = 0;
for(int i = 0; i <= n; ++i){
last[i][0] = INF;
pref[0][i] = INF;
for(int j = 1; j <= n; ++j) last[i][j] = min(last[i][j - 1] + mn[0], dp[0][i][j][1]);
for(int j = 1; j <= n; ++j) pref[j][i] = min(pref[j - 1][i], dp[0][j][i][1]);
}
for(int h = 1; h <= a[n][0] + 2; ++h){
for(int j = 0; j <= p[h - 1]; ++j){
for(int i = 0; i <= p[h] - p[h - 1] + j; ++i){
int sz = p[h] - p[h - 1] + j - i;
dp[h][i][sz][1] = min(dp[h][i][sz][1], last[j][sz]);
}
}
for(int i = 0; i <= p[h]; ++i){
for(int mx = 1; mx <= n; ++mx){
if(p[h - 1] - p[h] + i + mx - 1 >= 0){
dp[h][i][mx][1] = min(dp[h][i][mx][1], pref[min(p[h - 1] - p[h] + i + mx - 1, (ll)n)][mx]);
}
}
}
for(int i = 0; i <= p[h]; ++i) for(int mx = 0; mx <= p[h]; ++mx) dp[h][i][mx][1] = min(INF, dp[h][i][mx][1] + i * k);
for(int j = 0; j <= p[h - 1]; ++j){
int sz = j + p[h] - p[h - 1];
for(int mx = sz; mx <= p[h - 1]; ++mx){
dp[h][0][mx][1] = min(dp[h][0][mx][1], dp[h - 1][j][mx][1]);
}
}
if(p[h] > p[h - 1]){
dp[h][p[h] - 1][1][1] = min(dp[h][p[h] - 1][1][1], dp[h - 1][p[h - 1]][0][0] + (p[h] - 1) * k);
}
dp[h][p[h]][0][0] = min(dp[h][p[h]][0][0], dp[h - 1][p[h - 1]][0][0] + p[h] * k);
for(int i = 0; i <= n; ++i){
last[i][0] = INF;
pref[0][i] = INF;
for(int j = 1; j <= n; ++j) last[i][j] = min(last[i][j - 1] + mn[h], dp[h][i][j][1]);
for(int j = 1; j <= n; ++j) pref[j][i] = min(pref[j - 1][i], dp[h][j][i][1]);
}
}
for(int h = a[n][0]; h <= a[n][0] + 2; ++h){
for(int mx = 1; mx <= n; ++mx) ans = min(ans, dp[h][0][mx][1]);
}
cout << ans;
}
int main(){
cin.tie(0); ios::sync_with_stdio(0);
int tt = 1, aa;
// freopen("in.txt", "r", stdin);
// freopen("out.txt", "w", stdout);
while(tt--){
solve();
en;
}
cerr<<"time taken : "<<(float)clock()/CLOCKS_PER_SEC<<" seconds\n";
return 0;
}
