import sys
input = sys.stdin.readline

def main():
    N, M = map(int, input().split())
    
    l = [list(map(int, input().split())) for l in range(M)]

    lout = [0] * (N+1)
    lin = [0] * (N+1)
    lans = [0] * (N+1)

    for i in range(M):
        lout[l[i][0]] += 1
        lin[l[i][1]] += 1

    for i in range(1, N + 1):
        lans[i] = lout[i] - lin[i]

    lpos = sum([i for i in lans if i > 0])
    lneg = sum([i for i in lans if i < 0])

    if lpos == -lneg:
        print(lpos - 1)
    else:
        print(-1)

if __name__ == '__main__':
    main()