#include <bits/stdc++.h>
using namespace std;
#define sz(v) int(std::size(v))
#define FOR(i,a,b) for (int i = a; i < b; ++i)
#define ROF(i,a,b) for (int i = b-1; i >= a; --i)
typedef long long i64;
typedef vector<int> vi;
const i64 INF = 1e18;
template<class T> void chmin(T &x, T y) {
x = min(x, y);
}
int N; i64 K;
vi H, C;
void setup() {
cin >> N >> K;
H.resize(N), C.resize(N);
FOR(i,0,N) cin >> H[i] >> C[i];
}
i64 S(i64 N) { return N*(N+1)/2; }
void solve() {
map<int, int> num, mnC;
vi h;
FOR(i,0,N) {
++num[H[i]];
if (!mnC.count(H[i])) mnC[H[i]] = C[i];
else chmin(mnC[H[i]], C[i]);
h.push_back(H[i]), h.push_back(H[i]+1), h.push_back(H[i]+2);
}
sort(begin(h), end(h)), h.erase(unique(begin(h), end(h)), end(h));
vector dp(N+1, vector(N+1, INF)), ndp(N+1, vector(N+1, INF));
int mn = mnC[h.at(0)];
dp[1][num[h.at(0)]-1] = 0;
FOR(t,1,sz(h)) {
int c = num[h[t]];
ndp = vector(N+1, vector(N+1, INF));
FOR(i,0,N+1) FOR(j,0,N+1) if (dp[i][j] < INF)
chmin(ndp[i][max(0, j+c-i)], dp[i][j]+j*K);
FOR(i,1,N+1) FOR(j,0,N) chmin(ndp[i][j], ndp[i-1][j+1]+mn);
dp = ndp;
ndp = vector(N+1, vector(N+1, INF));
if (mnC.count(h[t])) chmin(mn, mnC[h[t]]);
i64 l = t+1 < sz(h) ? h[t+1]-h[t]-1 : i64(1e14);
FOR(i,0,N+1) FOR(j,0,N+1) if (dp[i][j] < INF) {
if (j-l*i >= 0) chmin(ndp[i][j-l*i], dp[i][j] + K * (S(l)*i + l*(j-l*i)));
else chmin(ndp[i][0], dp[i][j] + K * (S(j/i)*i + (j%i)*(j/i+1)));
}
dp = ndp;
}
i64 ans = INF;
FOR(j,1,N+1) chmin(ans, dp[j][0]);
cout << ans << '\n';
}
int main() {
ios::sync_with_stdio(0); cin.tie(0);
setup();
solve();
}
