int n; cin >> n; string s; cin >> s; for (char c = 'A'; c <= 'Z'; c++) { int first = n; int last = -1; for (int i = 0; i < n; i++) { if (s[i] == c) { first = min(first, i); last = max(last, i); } } for (int i = first; i <= last; i++) { if (s[i] != c) { cout << "NO\n"; return; } } } cout << "YES\n";