import sys

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

    balance = [0] * (N + 1)  # 1-based indexing

    for _ in range(M):
        u = int(input[idx])
        idx += 1
        v = int(input[idx])
        idx += 1
        balance[u] += 1
        balance[v] -= 1

    P = 0
    N_count = 0
    S_plus = 0
    for i in range(1, N+1):
        b = balance[i]
        if b > 0:
            P += 1
            S_plus += b
        elif b < 0:
            N_count += 1

    # Check if already satisfies Eulerian trail condition
    if (P == 0 and N_count == 0) or (P == 1 and N_count == 1 and S_plus == 1):
        print(0)
        return

    # Check if P and N are in the correct relationship
    if not ((P == N_count) or (P == N_count + 1) or (N_count == P + 1)):
        print(-1)
        return

    # Calculate E
    E = max(P-1, N_count-1)
    if (S_plus - E) in (0, 1):
        print(E)
    else:
        print(-1)

if __name__ == "__main__":
    main()