HDU2647 topsort

#include<bits/stdc++.h>
using namespace std;
vector<int> mp[20005];
int n,m;
int a,b;
int in[20005];
int L[20005];
int flag;
int re;
struct node
{
    int sum;
};
queue<node>q;
struct node N,now;
int main()
{
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        for(int i=1;i<=n;i++)
            mp[i].clear();
        memset(in,0,sizeof(in));
        memset(L,0,sizeof(L));
        for(int i=0;i<m;i++)
        {
            scanf("%d%d",&a,&b);
            mp[b].push_back(a);
            in[a]++;
        }
        for(int i=1;i<=n;i++)
        {
            if(in[i]==0)
            {
                N.sum=i;
                q.push(N);
            }
        }
        int jishu=n;
        int temp;
           while(!q.empty())
            {
                now=q.front();
                q.pop();
                jishu--;
                temp=L[now.sum];
                //cout<<now.sum<<endl;
                for(unsigned int i=0;i<mp[now.sum].size();i++)
                {
                     if(--in[mp[now.sum][i]]==0)
                      {
                          N.sum=mp[now.sum][i];
                          L[N.sum]=temp+1;
                          q.push(N);
                      }
                }
            }
            re=0;
            if(jishu>0)
                printf("-1\n");
            else
            {
                for(int i=1;i<=n;i++)
                    re+=L[i];
                printf("%d\n",re+888*n);
            }
}
    return 0;
}