#include <bits/stdc++.h>

using namespace std;

#define MAXN 5005
int n, A[MAXN];
int dp[MAXN];

int main(){
    cin >> n;
    for (int i = 1; i <= n; i++) cin >> A[i];
    dp[1] = 0;
    for (int i = 2; i <= n; i++){
        dp[i] = 100000000;
        for (int j = 1; j < i; j++)
            if (i - j <= A[j])
                dp[i] = min(dp[i], dp[j] + 1);
    }
    cout << dp[n] << "\n";
}