Olymp

#include <iostream>
#include <vector>
#include <algorithm>

size_t n, m;
std::vector<std::vector<unsigned >> adj;
std::vector<int8_t> color;

void dfs(unsigned u, int8_t c) {
    color[u] = c;
    for(unsigned v: adj[u]) {
        if (color[v] == 0) {
            dfs(v, 3 - c);
        } else if (color[v] == c) {
            std::cout << -1;
            exit(0);
        }
    }
}

void solve() {
    for(size_t u = 0u; u < n; ++u) {
        if (color[u] == 0) {
            dfs(u, 1);
        }
    }

    std::cout << count_if(color.begin(), color.end(), [](int8_t e){ return e == 1;}) << '\n';
    for(size_t i = 0u; i < n; ++i) {
        if (color[i] == 1) {
            std::cout << i + 1 << ' ';
        }
    }
}

void read_data() {
    std::cin >> n >> m;
    adj.resize(n, std::vector<unsigned >());
    color.assign(n, 0);

    for(size_t i = 0; i < m; ++i) {
        unsigned u, v;
        std::cin >> u >> v;
        --u, --v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }
}


int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(0);
    std::cout.tie(0);

    read_data();
    solve();

    return 0;
}