HDU-2647-Reward

#include <iostream>
#include <memory.h>
#include <vector>
#include <map>
#include <algorithm>
#include <cstdio>
#include <math.h>
#include <queue>

using namespace std;

typedef long long LL;

const int MAXN = 1e4 + 10;

vector<int> Group[MAXN];
int in[MAXN];
int value[MAXN];

int n, m;

bool topo()
{
    queue<int> que;
    int cnt = 0;
    for (int i = 1;i <= n;i++)
        if (in[i] == 0)
        {
            que.push(i);
            value[i] = 888;
        }

    while (!que.empty())
    {
        int now = que.front();
        cnt++;
        que.pop();
        for (auto x : Group[now])
        {
            if (--in[x] == 0)
            {
                que.push(x);
                value[x] = value[now] + 1;
            }
        }
    }

    if (cnt == n)
        return true;
    return false;

}

int main()
{
    int l, r;
    while (~scanf("%d%d", &n, &m))
    {
        for (int i = 1;i <= n;i++)
            Group[i].clear();
        memset(in, 0, sizeof(in));

        for (int i = 1; i <= m; i++)
        {
            scanf("%d%d", &l, &r);
            Group[r].push_back(l);
            in[l]++;
        }

        int res = 0;
        if (topo())
        {
            for (int i = 1;i <= n;i++)
                res += value[i];
        }
        else
            res = - 1;

        cout << res << endl;
    }


    return 0;
}