#include<bits/stdc++.h>
#define M 5004
using namespace std;
 
int n, a[M], d[M], dp[M][M];
 
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);     cout.tie(0);
 
    cin >> n;
    for(int i = 1; i <= n; i++)
        cin >> a[i];
    for(int i = 1; i < n; i++)
        d[i] = a[i + 1] - a[i];
    int ans = 0;
    for(int i = 1; i < n - 4; i++)
    {
        for(int j = i + 4; j < n; j++)
        {
            if(d[i] == d[j])
            {
                dp[i][j] = min(dp[i - 1][j - 1] + 1, j - i - 1);
                ans = max(ans, dp[i][j]);
            }
        }
    }
    if(ans >= 4)    cout << ans + 1;
    else    cout << 0;
}
 

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