import sys
sys.setrecursionlimit(5*10**5)
input = sys.stdin.readline
from collections import defaultdict, deque, Counter
from heapq import heappop, heappush
from bisect import bisect_left, bisect_right
from math import gcd

dout = defaultdict(int)
din = defaultdict(int)

n,m = map(int,input().split())
for i in range(m):
    a,b = map(int,input().split())
    dout[b-1]+=1
    din[a-1]+=1

cnt1=0
cnt2=0
for i in range(n):
    if dout[i] == din[i]:
        continue
    elif dout[i] > din[i]:
        cnt1+= dout[i] - din[i]
    else:
        cnt2+= din[i] - dout[i]

if cnt1 != cnt2:
    print(-1)
elif cnt1+cnt2 == 0:
    print(0)
else:
    print((cnt1+cnt2)//2-1)