#include <bits/stdc++.h>
using namespace std;
template <typename T> using vec = vector<T>;
template <typename T> using vvec = vec<vec<T>>;
template <typename T> using lqueue = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using gqueue = priority_queue<T, vector<T>, less<T>>;

int main() {
    constexpr int inf = INT_MAX / 4;

    int c, n; cin >> c >> n;
    vec<int> a(n);
    for (auto &e : a) cin >> e;

    vec<int> dist(c + 1, inf);
    lqueue<array<int, 2>> que;

    dist[0] = 0;
    que.push({ 0, 0 });
    while (!que.empty()) {
        auto [cost, vertex] = que.top(); que.pop();

        for (auto d : a) {
            int next = vertex + d;
            if (c < next || dist[next] <= dist[vertex] + 1) continue;
            dist[next] = dist[vertex] + 1;
            que.push({ dist[next], next });
        }
    }

    if (dist[c] == inf) {
        cout << -1 << endl;
    }
    else {
        cout << dist[c] << endl;
    }
}