題目描述

Bessie and her sister Elsie want to travel from the barn to theirfavorite field, such that they leave at exactly the same time from the barn,and also arrive at exactly the same time at their favorite field. The farm is acollection of N fields (1 <= N <= 100) numbered 1..N, where field 1contains the barn and field N is the favorite field. The farm is built on theside of a hill, with field X being higher in elevation than field Y if X <Y. An assortment of M paths connect pairs of fields. However, since each pathis rather steep, it can only be followed in a downhill direction. For example,a path connecting field 5 with field 8 could be followed in the 5 -> 8direction but not the other way, since this would be uphill. Each pair offields is connected by at most one path, so M <= N(N-1)/2. It might takeBessie and Elsie different amounts of time to follow a path; for example,Bessie might take 10 units of time, and Elsie 20. Moreover, Bessie and Elsieonly consume time when traveling on paths between fields -- since they are in ahurry, they always travel through a field in essentially zero time, neverwaiting around anywhere. Please help determine the shortest amount of timeBessie and Elsie must take in order to reach their favorite field at exactlythe same moment.

輸入

The first input line contains N and M, separated by a space. Eachof the following M lines describes a path using four integers A B C D, where Aand B (with A < B) are the fields connected by the path, C is the timerequired for Bessie to follow the path, and D is the time required for Elsie tofollow the path. Both C and D are in the range 1..100.

輸出

A single integer, giving the minimum time required for Bessie andElsie to travel to their favorite field and arrive at the same moment. If thisis impossible, or if there is no way for Bessie or Elsie to reach the favoritefield at all, output the word IMPOSSIBLE on a single line.

樣例輸入

33

1 3 12

1 2 12

2 3 12

樣例輸出

2

提示:Bessie is twice as fast as Elsie on each path, butif Bessie takes the path 1->2->3 and Elsie takes the path 1->3 theywill arrive at the same time.

#include <cstdio>
#define N 10005
int n,m,x,y,xx,yy;
int cnt,first[N],next[N],v[N],w[N];
int first1[N],next1[N],v1[N],w1[N];
int q[N*500],q1[N*500],head,tail;
bool ans[N],ans1[N];
using namespace std;
inline void spfa(int x)
{
    head=0;
    tail=1;
    q[1]=x;
    while (head<tail)
    {
        head++;
        for (int i=first[q[head]];i;i=next[i])
        {
            tail++;
            q[tail]=v[i];
            q1[tail]=q1[head]+w[i];
            if (v[i]==n) ans[q1[tail]]=1;
        }
    }
}
inline void spfa1(int x)
{
    head=0;
    tail=1;
    q[1]=x;
    while (head<tail)
    {
        head++;
        for (int i=first1[q[head]];i;i=next1[i])
        {
            tail++;
            q[tail]=v1[i];
            q1[tail]=q1[head]+w1[i];
            if (v1[i]==n) ans1[q1[tail]]=1;
        }
    }
}
int main()
{
    scanf("%d%d",&n,&m);
    for (int i=1;i<=m;i++)
    {
        scanf("%d%d%d%d",&x,&y,&xx,&yy);
        next[++cnt]=first[x];
        first[x]=cnt;
        v[cnt]=y;
        w[cnt]=xx;
        next1[cnt]=first1[x];
        first1[x]=cnt;
        v1[cnt]=y;
        w1[cnt]=yy;
    }
    spfa(1);
    spfa1(1);
    for (int i=1;i<10005;i++)
      if (ans[i]&&ans1[i])
      {
      	printf("%d\n",i);
      	return 0;
      }
    printf("IMPOSSIBLE\n");
    return 0;
}