#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int, int> pii;

#define eb emplace_back
#define pb push_back
#define all(s) (s).begin(), (s).end()
#define rep(i, a, b) for (int i = (a); i <= (b); ++i)
#define per(i, a, b) for (int i = (a); i >= (b); --i)

constexpr int N = 2007;

int n, m, scc, bel[N];

bool vis[N];

vector<int> e[N], re[N], order;

void dfs1(int u) {
    vis[u] = true;
    for (auto v : e[u]) if (!vis[v]) dfs1(v);
    order.eb(u);
}

void dfs2(int u) {
    bel[u] = scc;
    for (auto v : re[u]) if (!bel[v]) dfs2(v);
}

inline void work() {
    scc = 0;
    order.clear();
    rep(i, 1, (n << 1 | 1)) {
        e[i].clear();
        re[i].clear();
        vis[i] = false;
        bel[i] = 0;
    }
    // 2 * i : not advance
    // 2 * i + 1 : advance
    e[2].eb(3);
    rep(i, 1, m) {
        int x, y;
        cin >> x >> y;
        x = (x < 0 ? ((-x) << 1) : (x << 1 | 1));
        y = (y < 0 ? ((-y) << 1) : (y << 1 | 1));
        e[x ^ 1].eb(y); re[y].eb(x ^ 1);
        e[y ^ 1].eb(x); re[x].eb(y ^ 1);
    }
    rep(u, 2, (n << 1 | 1)) if (!vis[u]) dfs1(u);
    reverse(all(order));
    for (auto u : order) if (!bel[u]) {++scc; dfs2(u);}
    rep(i, 1, n) if (bel[i << 1] == bel[i << 1 | 1]) {puts("no"); return;}
    puts("yes");
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    while (cin >> n >> m) work();
    return 0;
}