#include <bits/stdc++.h>
using namespace std;
using ll = long long; using pii = pair<ll,ll>;
const ll Nm = 305; const ll Hm = 700; const ll INF = 2e18;
ll dp[Hm][Nm][Nm];
//height being processed
//current flow
//available pull requests
int main() {
ios_base::sync_with_stdio(false); cin.tie(0);
ll N,K; cin >> N >> K;
ll ans = 0;
ll H0[N]; ll C0[N];
for (ll i=0;i<N;i++) {
cin >> H0[i] >> C0[i];
}
ll n0[Hm];
ll psn0[Hm+1];
psn0[0]=0;
ll cmin[Hm];
for (ll i=0;i<Hm;i++) {
cmin[i]=INF;
n0[i]=0;
}
for (ll i=0;i<N;i++) {
n0[H0[i]]++;
cmin[H0[i]]=min(C0[i],cmin[H0[i]]);
}
ll hmin = INF;
for (ll h=0;h<Hm;h++) {
if (n0[h]!=0) {
hmin = h;
n0[h+1]+=(n0[h]-1);
ans += K*(n0[h]-1);
n0[h]=1;
break;
}
}
for (ll h=0;h<(Hm-hmin);h++) {
n0[h]=n0[h+hmin];
cmin[h]=cmin[h+hmin];
}
for (ll h=0;h<Hm;h++) {
psn0[h+1]=psn0[h]+n0[h];
}
for (ll i=0;i<Hm;i++) {
for (ll j=0;j<Nm;j++) {
for (ll k=0;k<Nm;k++) {
dp[i][j][k]=INF;
}
}
}
dp[0][0][1]=0;
ll rmin = INF; //rolling cmin
for (ll h=0;h<(Hm-1);h++) {
ll n0v = n0[h];
for (ll f=0;f<Nm;f++) {
for (ll p=1;p<Nm;p++) {
if (dp[h][f][p]>=INF) {
continue;
}
ll xn = n0v+f;
if (xn>p) {
dp[h+1][xn-p][p]=min(dp[h+1][xn-p][p],dp[h][f][p]+K*(xn-p));
} else {
dp[h+1][0][p]=min(dp[h+1][0][p],dp[h][f][p]);
}
}
}
rmin = min(rmin,cmin[h]);
for (ll f=0;f<Nm;f++) {
for (ll p=0;p<(Nm-1);p++) {
dp[h+1][f][p+1]=min(dp[h+1][f][p+1],dp[h+1][f][p]+rmin);
}
}
}
//process DP
ll ansf = INF;
for (ll j=0;j<Nm;j++) {
ansf = min(ansf,dp[Hm-1][0][j]);
}
cout << (ans+ansf)<<"\n";
}
