#include using namespace std; using ll = long long; using VI = vector; using VL = vector; using VVI = vector>; using VVL = vector>; using PII = std::pair; using PLL = std::pair; #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 void show1dvec(const vector &v) { if (v.size() == 0) return; int n = v.size() - 1; rep(i, n) cout << v[i] << wsp; cout << v[n] << rt; ; return; } template void show2dvec(const vector> &v) { int n = v.size(); rep(i, n) show1dvec(v[i]); } template void show1dpair(const vector> &v) { int n = v.size(); rep(i, n) cout << v[i].first << wsp << v[i].second << rt; return; } template void pairzip(const vector> &v, vector &t, vector &s) { int n = v.size(); rep(i, n) { t.push_back(v[i].first); s.push_back(v[i].second); } return; } template void maxvec(vector &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 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 &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 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 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; }