#include <iostream>
using namespace std;
bool check(int l,int r,int a[])
{
int k = (l+r)/2;
for (int i = l;i <= k;i++)
{
if (a[i] != a[r-i])
return false;
}
return true;
}
int main()
{
int n;cin >> n;
int m = 0;
int a[n];
for (int i = 0;i < n;i++)
cin >> a[i];
for (int i = 0;i < n-1;i++)
{
for (int j = i+1;j < n;j++)
{
if (a[i] == a[j])
if (a[i+1] == a[j-1])
if (check(i,j,a) == true)
m = max(m,j-i+1);
}
}
cout << m;
}