n,m = map(int,input().split())
graph=[[] for _ in range(n)]
for i in range(m):
    a,b = map(int,input().split())
    graph[a-1].append(b-1)

c1=0
c2=0
for i in range(n):
    if len(graph[i])%2==1:
        c1+=1
    else:
        c2+=1

if c1-2 > 0:
    print(abs(c1-2))
elif c1-2 == 0 or c1 == 0:
     print(0)
else:
    print(-1)