#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
using VVI = vector<vector<int>>;
using VVL = vector<vector<ll>>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); i++)
#define rep3(i, s, n, d) for (int i = (s); i < (int)(n); i += (d))
#define allpt(v) (v).begin(), (v).end()
#define allpt_c(v) (v).cbegin(), (v).cend()
#define allpt_r(v) (v).rbegin(), (v).rend()

const int mod = 1e9 + 7, mod2 = 998244353;
const ll inf = 1e18;

const string wsp = " ";
const string tb = "\t";
const string rt = "\n";

template <typename T>
void show1dvec(const vector<T> &v)
{
    if (v.size() == 0)
        return;
    int n = v.size() - 1;
    rep(i, n) cout << v[i] << wsp;
    cout << v[n] << rt;
    ;
    return;
}

template <typename T>
void show2dvec(const vector<vector<T>> &v)
{
    int n = v.size();
    rep(i, n) show1dvec(v[i]);
}

template <typename T, typename S>
void show1dpair(const vector<pair<T, S>> &v)
{
    int n = v.size();
    rep(i, n) cout << v[i].first << wsp << v[i].second << rt;
    return;
}

template <typename T, typename S>
void pairzip(const vector<pair<T, S>> &v, vector<T> &t, vector<T> &s)
{
    int n = v.size();
    rep(i, n)
    {
        t.push_back(v[i].first);
        s.push_back(v[i].second);
    }
    return;
}

template <typename T>
void maxvec(vector<T> &v)
{
    T s = v[0];
    int n = v.size();
    rep(i, n - 1)
    {
        if (s > v[i + 1])
        {
            v[i + 1] = s;
        }
        s = v[i + 1];
    }
}

template <typename T, typename S>
bool myfind(T t, S s)
{
    return find(t.cbegin(), t.cend(), s) != t.cend();
}

bool check(int y, int x, int h, int w)
{
    return 0 <= y && y < h && 0 <= x && x < w;
}

VL cumsum(const VL &v)
{
    int s = 0;
    VL ret = {0};

    rep(i, v.size())
    {
        s += v[i];
        ret.push_back(s);
    }
    return ret;
}

struct point
{
    int parent = -1;
    int depth = 0;
};

void dfs(int &c, int s, const VVI &connect, VI &order_list, vector<point> &points, VI &order_index)
{
    order_index[s] = c;
    c++;
    order_list.push_back(s);

    for (auto t : connect[s])
        if (t != points[s].parent)
        {
            points[t].parent = s;
            points[t].depth = points[s].depth + 1;
            dfs(c, t, connect, order_list, points, order_index);
            c++;
            order_list.push_back(s);
        }
    return;
}

class RangeMinorMaxorSumQuery // 0-index
{
    int const intmax = 2147483647;
    int const intmin = 0;
    vector<int> sgt;
    int n;
    int k;

public:
    RangeMinorMaxorSumQuery(int n1, int f = -1)
    {
        if (f == -1)
            f = intmax;
        else if (f == 0)
            f = intmin;
        int na = 1;
        int ka = 0;
        while (na < n1)
        {
            na *= 2;
            ka++;
        }
        for (int i = 0; i < 2 * na; i++)
            sgt.push_back(f);
        n = na;
        k = ka;
    }

    void update_min(int i, int x)
    {
        i += n;
        sgt[i] = x;
        while (i > 1)
        {
            i /= 2;
            sgt[i] = min(sgt[2 * i], sgt[2 * i + 1]);
        }
    }

    int getmin(int a, int b, int k = 1, int l = 0, int r = -1) //閉区間 l <= x < r とする
    {
        if (r == -1)
            r = n;
        if (r <= a || b <= l)
            return intmax;
        if (a == l && b == r)
            return sgt[k];
        else
            return min(getmin(a, min(b, (l + r) / 2), 2 * k, l, (l + r) / 2), getmin(max(a, (l + r) / 2), b, 2 * k + 1, (l + r) / 2, r));
    }

    void printsegtree()
    {
        for (int i = 0; i < 2 * n; i++)
        {
            cout << sgt[i] << " ";
        }
        cout << endl;
    }
};


int main()
{

#ifdef DEBUG
    cout << "DEBUG MODE" << endl;
    ifstream in("input.txt"); //for debug
    cin.rdbuf(in.rdbuf());    //for debug
#endif

    int n;
    cin >> n;
    VI kamo(n);
    set<int> diff;
    rep(i, n) cin >> kamo[i];
    sort(allpt(kamo));
    rep(i, n - 1) diff.insert(kamo[i + 1] - kamo[i]);
    cout << (diff.size() == 1 && *diff.cbegin() != 1 ? "YES" : "NO") << rt;

    return 0;
}