#include <bits/stdc++.h>

using namespace std;
int Max;
int x, y, len = 1;

int main() {
    int n;
    /*
    10
    1 5 6 2 3 4 5 6 8 9
    */
    cin >> n; // 10

    cin >> x; // 1

    for (int i = 2; i <= n; i++) {
        cin >> y;
        if (y == x + 1)
            len++, Max = max(Max, len);
        else
            len = 1;

        //这句话需要重点理解
        x = y;
    }
    cout << Max << endl;
    return 0;
}