n个人，m种关系，输入a，b，表示a比b的工资高，每个人的基本工资都为888，至少需要准备多少钱发工资。
// #pragma GCC optimize(2)
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <queue>
#include <cmath>
#include <string>
#include <vector>
#include <stack>
#include <map>
#include <sstream>
#include <cstring>
#include <set>
#include <cctype>
#include <bitset>
#define IO \ios::sync_with_stdio(false); \// cout.tie(0);
using namespace std;
// int dis[8][2] = {0, 1, 1, 0, 0, -1, -1, 0, 1, -1, 1, 1, -1, 1, -1, -1};
typedef unsigned long long ULL;
typedef long long LL;
typedef pair<int, int> P;
const int maxn = 1e5 + 10;
const int maxm = 2e5 + 10;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int inf = 0x3f3f3f3f;
const LL mod = 1e9 + 7;
const double pi = acos(-1);
struct Edge
{int next, to;
} e[maxm];
int n, m, k;
int in[maxn], dep[maxn];
int head[maxn];
void add(int u, int v)
{e[k].next = head[u];e[k].to = v;head[u] = k++;
}
LL topo()
{queue<int> q;LL ans = 0;int cnt = 0;for (int i = 1; i <= n; i++){if (in[i] == 0){q.push(i);cnt++;dep[i] = 0;}}while (!q.empty()){int u = q.front();q.pop();ans += LL(888 + dep[u]);for (int i = head[u]; i != -1; i = e[i].next){int v = e[i].to;in[v]--;if (!in[v]){cnt++;dep[v] = dep[u] + 1;q.push(v);}}}if (cnt == n)return ans;elsereturn -1;
}
int main()
{
#ifdef ONLINE_JUDGE
#elsefreopen("in.txt", "r", stdin);// freopen("out.txt", "w", stdout);
#endifIO;int x, y;while (cin >> n >> m){k = 0;for (int i = 0; i <= n; i++){head[i] = -1, in[i] = 0, dep[i] = 0;}for (int i = 0; i < m; i++){cin >> y >> x;in[y]++;add(x, y);}cout << topo() << "\n";}return 0;
}