//四則演算 #pragma GCC target("avx") //並列計算 #pragma GCC optimize("O3") //条件分岐を減らす #pragma GCC optimize("unroll-loops") //浮動小数点演算 #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native") #include using namespace std; using ll = long long; using ld = long double; using VI = vector; using VL = vector; using VS = vector; template using PQ = priority_queue, greater>; #define FOR(i,a,n) for(int i=(a);i<(n);++i) #define eFOR(i,a,n) for(int i=(a);i<=(n);++i) #define rFOR(i,a,n) for(int i=(n)-1;i>=(a);--i) #define erFOR(i,a,n) for(int i=(n);i>=(a);--i) #define each(i, a) for(auto &i : a) #define SORT(i) sort(i.begin(),i.end()) #define rSORT(i) sort(i.rbegin(),i.rend()) #define fSORT(i,a) sort(i.begin(),i.end(),a) #define all(i) i.begin(),i.end() #define out(y,x) ((y)<0||h<=(y)||(x)<0||w<=(x)) #define line cout << "-----------------------------\n" #define ENDL(i,n) ((i) == (n) - 1 ? "\n" : " ") #define stop system("pause") constexpr ll INF = 1000000000; constexpr ll LLINF = 1LL << 60; constexpr ll mod = 1000000007; constexpr ll MOD = 998244353; constexpr ld eps = 1e-10; constexpr ld pi = 3.1415926535897932; templateinline bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; }return false; } templateinline bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; }return false; } inline void init() { cin.tie(nullptr); cout.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } templateinline istream& operator>>(istream& is, vector& v) { for (auto& a : v)is >> a; return is; } templateinline istream& operator>>(istream& is, deque& v) { for (auto& a : v)is >> a; return is; } templateinline istream& operator>>(istream& is, pair& p) { is >> p.first >> p.second; return is; } templateinline vector vec(size_t a) { return vector(a); } templateinline vector defvec(T def, size_t a) { return vector(a, def); } templateinline auto vec(size_t a, Ts... ts) { return vector(ts...))>(a, vec(ts...)); } templateinline auto defvec(T def, size_t a, Ts... ts) { return vector(def, ts...))>(a, defvec(def, ts...)); } inline void print() { cout << "\n"; } templateinline void print(const T& a, const Ts&... ts) { cout << a << " "; print(ts...); } templateinline void print(const vector& v) { for (int i = 0; i < v.size(); ++i)cout << v[i] << (i == v.size() - 1 ? "\n" : " "); } templateinline void print(const vector>& v) { for (auto& a : v)print(a); } inline string reversed(const string& s) { string t = s; reverse(all(t)); return t; } int main() { init(); int n; cin >> n; VI a(n); cin >> a; int ans = 0; if (a.front() == a.back()) { while (!a.empty() && a.front() == a.back()) { a.pop_back(); } if (!a.empty())++ans; } n = a.size(); map m; FOR(i, 0, n) { if (m[a[i]]) { print(-1); return 0; } m[a[i]] = true; for (int x = a[i]; i < n && a[i] == x; ++i); --i; } print(ans); return 0; }