USACO 2016 Open Platinum Landscaping

#include <cstdio>
#include <queue>
#include <vector>

using namespace std;

int main() {
    FILE *fin = fopen("landscape.in", "r");
    FILE *fout = fopen("landscape.out", "w");
    long long n, x, y, z;
    fscanf(fin, "%lld %lld %lld %lld", &n, &x, &y, &z);
    long long result = 0;
    priority_queue<long long> q1;
    priority_queue<long long> q2;
    for (int i = 0; i < n; i++) {
        int a, b;
        fscanf(fin, "%d %d", &a, &b);
        while (a > b) {
            long long cost = y;
            if (q2.size() && -q2.top() + i * z < y) {
                cost = -q2.top() + i * z;
                q2.pop();
            }
            result += cost;
            q1.push(i * z + cost);
            a--;
        }
        while (b > a) {
            long long cost = x;
            if (q1.size() && -q1.top() + i * z < x) {
                cost = -q1.top() + i * z;
                q1.pop();
            }
            result += cost;
            q2.push(i * z + cost);
            b--;
        }
    }
    fprintf(fout, "%lld\n", result);
    return 0;
}