POJ 2431 Expedition

#include<cstdio>
#include<cstring>
#include<cmath>
#include<queue>
#include<algorithm>
using namespace std;

int n;
struct Point
{
    long long x;
    long long sum;
}p[100000 + 10];
long long L, P;

bool cmp(const Point&a, const Point&b)
{
    return a.x<b.x;
}

int main()
{
    while (~scanf("%d", &n))
    {
        priority_queue<int>q;
        for (int i = 1; i <= n; i++) scanf("%lld%lld", &p[i].x, &p[i].sum);
        scanf("%lld%lld", &L, &P);
        for (int i = 1; i <= n; i++) p[i].x = L - p[i].x;
        sort(p + 1, p + 1 + n, cmp);
        if (p[n].x != L){ p[n + 1].x = L; p[n + 1].sum = 0; n++; }
        int ans = 0; long long pre = 0;
        for (int i = 1; i <= n; i++)
        {
            long long dis = p[i].x - pre;
            P = P - dis;
            if (p[i].x == L&&P >= 0) break;
            if (p[i].x == L)
            {
                while (P < 0 && (!q.empty()))
                {
                    P = P + q.top(); q.pop(); ans++;
                }
                if (P < 0) ans = -1; 
                break;
            }
            else
            {
                pre = p[i].x;
                if (P == 0) 
                {
                    q.push(p[i].sum);
                    P = P + q.top(); q.pop(); ans++;
                }
                else if (P < 0)
                {
                    while (P < 0 && (!q.empty()))
                    {
                        P = P + q.top(); q.pop(); ans++;
                    }
                    if (P < 0) { ans = -1; break; }
                    else if (P == 0)
                    {
                        q.push(p[i].sum);
                        P = P + q.top(); q.pop(); ans++;
                    }
                    else if (P>0) q.push(p[i].sum); 
                }
                else q.push(p[i].sum);
            }
        }
        printf("%d\n", ans);
    }
    return 0;
}