Codeforces 543 B. World Tour

#include<queue>
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>

using namespace std;

#define N 3001

int dis[N][N];

int s1,t1,s2,t2,l1,l2;

int n;

int tot;
int front[N],to[N*N],nxt[N*N];

int mi;

void read(int &x)
{
    x=0; char c=getchar();
    while(!isdigit(c)) c=getchar();
    while(isdigit(c)) { x=x*10+c-‘0‘; c=getchar(); }
}

void add(int u,int v)
{
    to[++tot]=v; nxt[tot]=front[u]; front[u]=tot;
    to[++tot]=u; nxt[tot]=front[v]; front[v]=tot;
}

void bfs(int s)
{
    int now,t;
    static queue<int>q;
    q.push(s);
    while(!q.empty())
    {
        now=q.front();
        q.pop();
        for(int i=front[now];i;i=nxt[i])
        {
            t=to[i];
            if(dis[s][t]==-1) 
            {
                dis[s][t]=dis[s][now]+1;
                q.push(t);
            }
        }
    }
}

void solve()
{
    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) continue;
            mi=min(mi,dis[s1][i]+dis[j][t1]+dis[s2][i]+dis[j][t2]+dis[i][j]);
            
        }
}

int main()
{
    int m;
    read(n); read(m);
    int u,v;
    for(int i=1;i<=m;++i)
    {
        read(u); read(v);
        add(u,v);
    }
    memset(dis,-1,sizeof(dis));
    for(int i=1;i<=n;++i) dis[i][i]=0;
    for(int i=1;i<=n;++i) bfs(i);
    read(s1); read(t1); read(l1);
    read(s2); read(t2); read(l2);
    if(dis[s1][t1]>l1 || dis[s2][t2]>l2)
    {
        printf("-1");
        return 0;
    }
    mi=dis[s1][t1]+dis[s2][t2];
    solve();
    swap(s1,t1);
    solve();
    printf("%d",m-mi);
}

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.