## https://yukicoder.me/problems/no/2403


class UnionFind:
    """
    UnionFindの基本的な処理を実装したクラス
    """

    def __init__(self, size):
        self.root = [i for i in range(size)]
        self.size = [1] * size

    def get_root(self, v):
        if v == self.root[v]:
            return v
        else:
            old_root = self.root[v]
            new_root = self.get_root(old_root)
            self.root[v] = new_root
            return new_root

    def merge(self, u, v):
        root_u = self.get_root(u)
        root_v = self.get_root(v)
        if root_u == root_v:
            return False

        if self.size[root_u] >= self.size[root_v]:
            self.size[root_u] += self.size[root_v]
            self.root[root_v] = root_u
            self.root[v] = root_u
        else:
            self.size[root_v] += self.size[root_u]
            self.root[root_u] = root_v
            self.root[u] = root_v
        return True
    

def main():
    N, M = map(int, input().split())
    in_degrees = [0] * N
    out_degrees = [0] * N

    uf = UnionFind(N)
    for _ in range(M):
        u, v = map(int, input().split())
        in_degrees[u - 1] += 1
        out_degrees[v - 1] += 1
        uf.merge(u - 1, v - 1)
    
    in_degree_total = 0
    out_degree_total = 0
    for i in range(N):
        if in_degrees[i] > out_degrees[i]:
            in_degree_total += in_degrees[i] - out_degrees[i]
        elif in_degrees[i] < out_degrees[i]:
            out_degree_total += out_degrees[i] - in_degrees[i]
    
    # 連結かどうか
    r_set = set()
    for i in range(N):
        r = uf.get_root(i)
        r_set.add(r)
    if len(r_set) > 1:
        print(-1)
        return

    if in_degree_total != out_degree_total:
        print(-1)
        return
    
    a = max(0, in_degree_total - 1)
    print(a)







if __name__ == "__main__":
    main()