#include<iostream>

using namespace std;

int main() {
    int n, ans = 0, c, b, max = 1;
    cin >> n >> b;

    for (int i = 0; i < n - 1; i++) {
        cin >> c;
        if (c == b + 1) max++;
        else max = 1;
        if (ans < max) ans = max;
        b = c;
    }
    cout << ans << endl;
    return 0;
}