import sys

def main():
    input = sys.stdin.read().split()
    ptr = 0
    N = int(input[ptr])
    ptr += 1
    M = int(input[ptr])
    ptr += 1

    out_degree = [0] * (N + 1)
    in_degree = [0] * (N + 1)

    for _ in range(M):
        u = int(input[ptr])
        ptr += 1
        v = int(input[ptr])
        ptr += 1
        out_degree[u] += 1
        in_degree[v] += 1

    d = [0] * (N + 1)
    for i in range(1, N+1):
        d[i] = out_degree[i] - in_degree[i]

    S = 0
    for i in range(1, N+1):
        S += abs(d[i])

    if S % 2 != 0:
        print(-1)
        return

    if S == 0:
        print(0)
        return

    option1 = S // 2

    if S >= 2 and (S - 2) % 2 == 0:
        option2 = (S - 2) // 2
        min_option = min(option1, option2)
    else:
        min_option = option1

    print(min_option)

if __name__ == "__main__":
    main()