#include <bits/stdc++.h>

using namespace std;

int main() {
    int n, k, x, m, ind, cur;
    vector<pair<int,int>> negative, positive;
    scanf("%d%d",&n,&k);
    for (int i = 0; i < n; i++) {
        scanf("%d%d",&x,&m);
        if (x < 0) {
            negative.push_back({-x,m});
        } else {
            positive.push_back({x,m});
        }
    }
    long long cnt = 0;
    sort(negative.begin(),negative.end());
    ind = negative.size() - 1;
    cur = 0;
    while(ind >= 0) {
        if (cur == 0) {
            cur = k;
            cnt += floor(negative[ind].second / cur)  * 2 * negative[ind].first;
            negative[ind].second %= cur;
            if (negative[ind].second == 0) ind--;
            if (ind == -1) break;
            cnt += 2 * negative[ind].first;
        }
        if (cur >= negative[ind].second) {
            cur -= negative[ind].second;
            negative[ind].second = 0;
            ind--;
        } else {
            negative[ind].second -= cur;
            cur = 0;
        }
    }
    sort(positive.begin(),positive.end());
    ind = positive.size() - 1;
    cur = 0;
    while(ind >= 0) {
        if (cur == 0) {
            cur = k;
            cnt += floor(positive[ind].second / cur) * 2 * positive[ind].first;
            positive[ind].second %= cur;
            if (positive[ind].second == 0) ind--;
            if (ind == -1) break;
            cnt += 2 * positive[ind].first;
        }
        if (cur >= positive[ind].second) {
            cur -= positive[ind].second;
            positive[ind].second = 0;
            ind--;
        } else {
            positive[ind].second -= cur;
            cur = 0;
        }
    }
    printf("%lld\n",cnt);
    return 0;
}