def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx += 1
    M = int(input[idx])
    idx += 1
    
    out_degree = [0] * (N + 1)
    in_degree = [0] * (N + 1)
    
    for _ in range(M):
        u = int(input[idx])
        idx += 1
        v = int(input[idx])
        idx += 1
        out_degree[u] += 1
        in_degree[v] += 1
    
    d = [0] * (N + 1)
    sum_d = 0
    for i in range(1, N + 1):
        d[i] = out_degree[i] - in_degree[i]
        sum_d += d[i]
    
    if sum_d != 0:
        print(-1)
        return
    
    S = 0
    for i in range(1, N + 1):
        if d[i] > 0:
            S += d[i]
    
    needed = S // 2
    print(needed)

if __name__ == '__main__':
    main()