#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using P = pair<int, int>;

vector<P> RLE(const vector<int>& v) {
    int tar = v.front();
    int cnt = 0;
    vector<P> res;
    for (int x : v) {
        if (tar == x) cnt++;
        else {
            res.emplace_back(tar, cnt);
            tar = x;
            cnt = 1;
        }
    }
    if (cnt > 0) res.emplace_back(tar, cnt);
    return res;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    vector<P> v = RLE(a);

    int sz = v.size();
    vector<int> dp(sz + 1, 0);
    dp[1] = v.front().second;
    for (int i = 1; i < sz; i++) {
        dp[i + 1] = max(dp[i] + max(v[i].second - 1, 0), dp[i - 1] + v[i].second);
    }
    cout << dp[sz] << "\n";
    return 0;
}