#include <bits/stdc++.h>
#define endl codeforces
#define ALL(v) std::begin(v), std::end(v)
#define ALLR(v) std::rbegin(v), std::rend(v)
using ll = std::int64_t;
using ull = std::uint64_t;
using pii = std::pair<int, int>;
using tii = std::tuple<int, int, int>;
using pll = std::pair<ll, ll>;
using tll = std::tuple<ll, ll, ll>;
template <typename T> using vec = std::vector<T>;
template <typename T> using vvec = vec<vec<T>>;
template <typename T> const T& var_min(const T &t) { return t; }
template <typename T> const T& var_max(const T &t) { return t; }
template <typename T, typename... Tail> const T& var_min(const T &t, const Tail&... tail) { return std::min(t, var_min(tail...)); }
template <typename T, typename... Tail> const T& var_max(const T &t, const Tail&... tail) { return std::max(t, var_max(tail...)); }
template <typename T, typename... Tail> void chmin(T &t, const Tail&... tail) { t = var_min(t, tail...); }
template <typename T, typename... Tail> void chmax(T &t, const Tail&... tail) { t = var_max(t, tail...); }
template <typename T> const T& clamp(const T &t, const T &low, const T &high) { return std::max(low, std::min(high, t)); }
template <typename T> void chclamp(T &t, const T &low, const T &high) { return t = clamp(t, low, high); }
template <typename T> T make_v(T init) { return init; }
template <typename T, typename... Tail> auto make_v(T init, std::size_t s, Tail... tail) { auto v = std::move(make_v(init, tail...)); return vec<decltype(v)>(s, v); }
template <typename T, std::size_t Head, std::size_t ...Tail> struct multi_dem_array { using type = std::array<typename multi_dem_array<T, Tail...>::type, Head>; };
template <typename T, std::size_t Head> struct multi_dem_array<T, Head> { using type = std::array<T, Head>; };
template <typename T, std::size_t ...Args> using mdarray = typename multi_dem_array<T, Args...>::type;
namespace init__ { struct InitIO { InitIO() { std::cin.tie(nullptr); std::ios_base::sync_with_stdio(false); std::cout << std::fixed << std::setprecision(30); } } init_io; }

namespace graph {

using Node = ll;
using Weight = ll;
using Edge = std::pair<Node, Weight>;

template <bool Directed>
struct Graph : public vvec<Edge> {
    using vvec<Edge>::vvec;

    void add_edge(Node f, Node t, Weight w = 1) {
        (*this)[f].emplace_back(t, w);
        if (!Directed) (*this)[t].emplace_back(f, w);
    }

    Graph<Directed> build_inv() const {
        Graph<Directed> ret(this->size());
        for (Node i = 0; i < this->size(); i++) {
            for (const Edge &e : (*this)[i]) {
                Node j;
                Weight w;
                std::tie(j, w) = e;
                if (!Directed && j < i) continue;
                ret.add_edge(j, i, w);
            }
        }

        return ret;
    }
};

template <typename Iterator>
class dst_iterator {
    Iterator ite;

public:
    dst_iterator(Iterator ite) : ite(ite) { }

    bool operator ==(const dst_iterator<Iterator> &oth) const {
        return ite == oth.ite;
    }

    bool operator !=(const dst_iterator<Iterator> &oth) const {
        return !(*this == oth);
    }

    bool operator <(const dst_iterator<Iterator> &oth) const {
        return ite < oth.ite;
    }

    bool operator >(const dst_iterator<Iterator> &oth) const {
        return ite > oth.ite;
    }

    bool operator <=(const dst_iterator<Iterator> &oth) const {
        return ite <= oth.ite;
    }

    bool operator >=(const dst_iterator<Iterator> &oth) const {
        return ite >= oth.ite;
    }

    const Node& operator *() {
        return ite->first;
    }

    const Node& operator *() const {
        return ite->first;
    }

    dst_iterator operator ++() {
        ++ite;
        return ite;
    }
};

class dst_iteration {
    using ite_type = vec<Edge>::const_iterator;
    const vec<Edge> &edges;

public:
    dst_iteration(const vec<Edge> &edges) : edges(edges) { }

    auto begin() const {
        return dst_iterator<ite_type>(edges.cbegin());
    }

    auto end() const {
        return dst_iterator<ite_type>(edges.cend());
    }
};

dst_iteration dst(const vec<Edge> &edges) {
    return dst_iteration(edges);
}

}

int main() {
    ll k, n;
    std::cin >> k >> n;
    vec<ll> a(n);
    for (ll &e : a) std::cin >> e;
    for (ll i = 0; i < n; i++) a.push_back(a[i]);
    ll ans = 5e15;
    for (ll i = 0; i < n; i++) {
        ll x = a[i];
        ll y = a[i + n - 1];
        ll diff = y - x;
        if (diff < 0) diff += k;
        chmin(ans, diff);
    }

    std::cout << ans << '\n';
    return 0;
}


Measured memory_bytes: 4864