int dp[200001][2] = {};
int dfs(int pos, vector<int>& v, vector<int>& mex, bool odd) {
    if (pos>=v.size() || pos >= v.size()-1) return 0;
    if (dp[pos][odd] != -1) return dp[pos][odd];
    int ans = 0, net = 0;
    if (v[pos+1] == mex[pos] && v[pos] > pos+1 && (mex[pos+1] == pos+3 || mex[pos+1] == pos+2)) net = 1;
    if (v[pos] <= pos+1 && mex[pos] == pos+2) net = -1;
    ans = max(ans, dfs(pos+2, v, mex, odd)+net);
    return dp[pos][odd] = ans;
}
void solve(){
    int n;
    cin >> n;
    vector<int> v(n, 0);
    for (int i = 0; i < n; i++) cin >> v[i];
    for (int i = 0; i < n; i++) dp[i][0] = dp[i][1] = -1;
    int ans = 0;
    int prev = 0;
    int pos = 1;
    vector<int> freq(n+1, 0);
    vector<int> mex(n, -1);
    for (int i = 0; i < n; i++) {
        freq[v[i]]++;
        while(pos<=n && freq[pos]) pos++;
        if (pos > i+1)prev++;
        mex[i] = pos;
    }
    for (int i = 0; i < n; i++) ans = max(ans, dfs(i, v, mex, i%2));
    cout << ans+prev << endl;
}