import sys
input = sys.stdin.readline

n, m = map(int, input().split())
parents = [i for i in range(n)]
rank = [0]*n

def find(x):
    if parents[x] != x:
        parents[x] = find(parents[x])
    return parents[x]

def union(x, y) :
    x = find(x)
    y = find(y)
    
    if x == y : return False 
    
    if rank[x] > rank[y] :
        parents[y] = x
    else :
        parents[x] = y
        if rank[x] == rank[y] :
            rank[y] += 1
    
    return True

ans = 0
for i in range(m) :
    a, b = map(int, input().split())
    
    if not union(a,b) :
        ans = i+1
        break

print(ans)