import math
from sys import stdin

n, q, *indata = map(int, stdin.read().split())
offset = 0
m = int(math.floor(math.sqrt(q)))
g = [[] for i in range(n+1)]
repeat = (q-1) // m + 1
cyclecheck = -1
for i in range(repeat):
    g2 = [[] for i in range(n+1)]
    for j in range(m):
        ind = i * m + j
        if ind >= q:
            break
        else:
            s, t = indata[ind*2]-1,indata[ind*2+1]-1
            g2[s].append(t)
    check = [0 for j in range(n)]
    for j in range(n):
        if check[j] == 0:
            que = [(j,0)]
            while que:
                now, inout = que.pop()
                if inout == 0:
                    if check[now] == 0:
                        que.append((now,1))
                        check[now] = 1
                        for k in g[now]:
                            if check[k] == 1:
                                cyclecheck = i
                                break
                            elif check[k] == 0:
                                que.append((k,0))
                        for k in g2[now]:
                            if check[k] == 1:
                                cyclecheck = i
                                break
                            elif check[k] == 0:
                                que.append((k,0))
                        if cyclecheck != -1:
                            break
                else:
                    check[now] = 2
        if cyclecheck != -1:
            break
    if cyclecheck != -1:
        break
    else:
        for j in range(m):
            ind = i * m + j
            if ind >= q:
                break
            else:
                s, t = indata[ind*2]-1,indata[ind*2+1]-1
                g[s].append(t)

if cyclecheck == -1:
    print("{}".format(-1))
    exit()

for i in range(m):
    ind = cyclecheck * m + i
    if ind >= q:
        break
    else:
        s, t = indata[ind*2]-1,indata[ind*2+1]-1
        g[s].append(t)
    check = [0 for j in range(n)]
    for j in range(n):
        if check[j] == 0:
            que = [(j,0)]
            while que:
                now, inout = que.pop()
                if inout == 0:
                    if check[now] == 0:
                        que.append((now,1))
                        check[now] = 1
                        for k in g[now]:
                            if check[k] == 1:
                                ans = cyclecheck * m + i + 1
                                print("{}".format(ans))
                                exit()
                            elif check[k] == 0:
                                que.append((k,0))
                else:
                    check[now] = 2