#include <iostream>
#include <cstring>

using namespace std;

const int N = 200010, INF = 0x3f3f3f3f;

int n, a[N], dp[N], ans;

int main() {
    scanf("%d", &n);
    for (int i = 1; i <= n; ++i) scanf("%d", &a[i]);

    memset(dp, 0x3f, sizeof(dp));
    dp[0] = 0;
    for (int i = 1; i <= n; ++i) {
        for (int j = i; j >= 0; --j) {
            if (j == i) {
                dp[j] = i;
                ans = i;
            } else {
                dp[j] = a[i] > dp[j] ? a[i] : INF;
                if (j >= 1) dp[j] = min(dp[j], dp[j - 1] + 1);
            }
            if (dp[j] != INF) {
                ans = j;
            }
        }
    }
    printf("%d\n", ans);

    return 0;
}