codeforces 554D Destroying Roads(最短路）

In some country there are exactly n cities and m bidirectional roads connecting the cities. Cities are numbered with integers from 1 to n. If cities a and b are connected by a road, then in an hour you can go along this road either from city a to city b, or from city b to city a. The road network is such that from any city you can get to any other one by moving along the roads.

You want to destroy the largest possible number of roads in the country so that the remaining roads would allow you to get from city s1 to city t1 in at most l1 hours and get from city s2 to city t2 in at most l2 hours.

Determine what maximum number of roads you need to destroy in order to meet the condition of your plan. If it is impossible to reach the desired result, print -1.

Input
The first line contains two integers n, m (1 ≤ n ≤ 3000, ) — the number of cities and roads in the country, respectively.

Next m lines contain the descriptions of the roads as pairs of integers ai, bi (1 ≤ ai, bi ≤ n, ai ≠ bi). It is guaranteed that the roads that are given in the description can transport you from any city to any other one. It is guaranteed that each pair of cities has at most one road between them.

The last two lines contains three integers each, s1, t1, l1 and s2, t2, l2, respectively (1 ≤ si, ti ≤ n, 0 ≤ li ≤ n).

Output
Print a single number — the answer to the problem. If the it is impossible to meet the conditions, print -1.

Examples
Input
5 4
1 2
2 3
3 4
4 5
1 3 2
3 5 2
Output
0
Input
5 4
1 2
2 3
3 4
4 5
1 3 2
2 4 2
Output
1
Input
5 4
1 2
2 3
3 4
4 5
1 3 2
3 5 1
Output
-1

题意：给出n个点，m条边权为1的无向边，破坏最多的道路，使得从s1到t1,s2到t2的距离不超过d1,d2

#include<bits/stdc++.h>
using namespace  std;
#define ll long long
#define pb push_back
#define inf 2099999999
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define rep1(i,a,b) for(int i=a;i>=b;i--)
const int N=3e3+10;
vector<int>G[N];
int dis[N][N];
int vis[N];
/**
    先把每个点到其他点的最短路求出来
    然后尽可能的找重边
    如果两个最短路都用到这个重边并且路径不超过限制
    那么要使用这条边
    之后除了最短路上的边,其他的边都是可摧毁的边
*/
int main()
{
    #ifdef LOCAL_DEFINE
        freopen("D://rush.txt", "r", stdin);
    #endif
    ios::sync_with_stdio(false),cin.tie(0);
    int n,m,a,b,s1,s2,t1,t2,l1,l2;
    cin>>n>>m;
    for(int i=0;i<m;i++)
    {
        cin>>a>>b;
        G[a].pb(b);
        G[b].pb(a);
    }
    cin>>s1>>t1>>l1>>s2>>t2>>l2;
    rep(i,1,n)
    {
        memset(vis,0,sizeof vis);
        vis[i]=1;
        queue<int>q;
        q.push(i);
        while(!q.empty())
        {
            int t= q.front();
            q.pop();
            for(int j=0;j<G[t].size();j++)
            {
                int y=G[t][j];
                if(!vis[y])
                {
                    dis[i][y]=dis[i][t]+1;
                    q.push(y);
                    vis[y]=1;
                }
            }
        }
    }
    if(dis[s1][t1]>l1||dis[s2][t2]>l2)
    {
        cout<<-1<<endl;
    }
    else
    {
        int ans=dis[s1][t1]+dis[s2][t2];
        for(int i=1;i<=n;i++)
        {
            for(int j=1;j<=n;j++)
            {
                if(dis[s1][i]+dis[i][j]+dis[j][t1]<=l1&&dis[s2][i]+dis[i][j]+dis[j][t2]<=l2)
                    ans=min(ans,dis[s1][i]+dis[i][j]+dis[j][t1]+dis[s2][i]+dis[j][t2]);
                if(dis[s1][i]+dis[i][j]+dis[j][t1]<=l1&&dis[t2][i]+dis[i][j]+dis[j][s2]<=l2)
                    ans=min(ans,dis[s1][i]+dis[i][j]+dis[j][t1]+dis[t2][i]+dis[j][s2]);
            }
        }
        cout<<m-ans<<endl;
    }
}