#include <bits/stdc++.h>
using namespace std;
int f[100002][3], dp[100002][3];
void solve() {
    int n,m=0; cin >> n;
    int a[n+1];
    for(int i=1;i<=n;i++) {
        cin >> a[i];
        f[i][a[i]]=f[i-1][a[i]]+1;
    }
    for(int i=n;i>=1;i--) {
        dp[i][a[i]]=dp[i+1][a[i]]+1;
        m=max(m,min(f[i-1][1],dp[i][2]));
        m=max(m,min(f[i-1][2],dp[i][1]));
        m=max(m,min(f[i][1],dp[i+1][2]));
        m=max(m,min(f[i][2],dp[i+1][1]));
    }
    cout << 2*m;
    return;
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    solve();
    return 0;
}