include [HTML_REMOVED]

include [HTML_REMOVED]

using namespace std;

const int N = 110, M = 2010;

int n, m;
int h[N], e[M], ne[M], idx;
bool st[N];

void add(int a, int b)
{
e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}

void dfs(int u)
{
st[u] = true;
for (int i = h[u]; i != -1; i = ne[i])
{
int j = e[i];
if (!st[j]) dfs(j);
}
}

int main()
{
cin >> n >> m;
memset(h, -1, sizeof h);
while (m–)
{
int a, b;
cin >> a >> b;
add(a, b);
}

dfs(1);

for (int i = 1; i <= n; i++)
{
    if (!st[i])
    {
        printf("-1\n");
        return 0;
    }
}
printf("1\n");

}

include [HTML_REMOVED]

include [HTML_REMOVED]

include [HTML_REMOVED]

using namespace std;

const int N = 110, M = 2010;

int n, m;
int h[N], e[M], ne[M], idx;
bool st[N];

void add(int a, int b)
{
e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}

void bfs()
{
queue[HTML_REMOVED] q;
q.push(1);
st[1] = true;

while (!q.empty())
{
    int t = q.front();
    q.pop();

    for (int i = h[t]; i != -1; i = ne[i])
    {
        int j = e[i];
        if (!st[j])
        {
            q.push(j);
            st[j] = true;
        }
    }
}

}

int main()
{
cin >> n >> m;
memset(h, -1, sizeof h);
while (m–)
{
int a, b;
scanf(“%d%d”, &a, &b);
add(a, b);
}
bfs();
for (int i = 1; i <= n; i++) // 遍历结点1~n
{
if (!st[i])
{
cout << -1 << endl;
return 0;
}
}
cout << 1 << endl;
}