import sys
input = sys.stdin.readline

class SCC:

    def __init__(self,n):
        self.n = n
        self.edges = []

    def csr(self):
        self.start = [0]*(self.n+1)
        self.elist = [0]*len(self.edges)
        for e in self.edges:
            self.start[e[0]+1] += 1
        for i in range(1,self.n+1):
            self.start[i] += self.start[i-1]
        counter = self.start[:]
        for e in self.edges:
            self.elist[counter[e[0]]] = e[1]
            counter[e[0]] += 1

    def add_edge(self,u,v):
        self.edges.append((u,v))

    def scc_ids(self):
        self.csr()
        n = self.n
        now_ord = group_num = 0
        visited = []
        low = [0]*n
        order = [-1]*n
        ids = [0]*n
        parent = [-1]*n
        stack = []
        for i in range(n):
            if order[i] == -1:
                stack.append(i)
                stack.append(i)
                while stack:
                    v = stack.pop()
                    if order[v] == -1:
                        low[v] = order[v] = now_ord
                        now_ord += 1
                        visited.append(v)
                        for i in range(self.start[v],self.start[v+1]):
                            to = self.elist[i]
                            if order[to] == -1:
                                stack.append(to)
                                stack.append(to)
                                parent[to] = v
                            else:
                                low[v] = min(low[v],order[to])
                    else:
                        if low[v] == order[v]:
                            while True:
                                u = visited.pop()
                                order[u] = n
                                ids[u] = group_num
                                if u == v:
                                    break
                            group_num += 1
                        if parent[v] != -1:
                            low[parent[v]] = min(low[parent[v]],low[v])
        for i,x in enumerate(ids):
            ids[i] = group_num-1-x

        return group_num,ids


    def scc(self):
        group_num,ids = self.scc_ids()
        groups = [[] for i in range(group_num)]
        for i,x in enumerate(ids):
            groups[x].append(i)

        return groups


    

n,q = map(int,input().split())
edges = [[int(x)-1 for x in input().split()] for i in range(q)]

def search(x):
    scc = SCC(n)
    for i in range(x):
        a,b = edges[i]
        scc.add_edge(a,b)

    groups = scc.scc()
    for g in groups:
        if len(g) > 1:
            return True
    return False


l = 0
r = q+1

while r > l + 1:
    m = (r+l)//2
    if search(m):
        r = m
    else:
        l = m

if l == q:
    if search(l):
        print(q)
    else:
        print(-1)
else:
    print(r)