#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <random>
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

const int MAX_N=1<<18;
const int INF=1100000000;
    
int mn[2*MAX_N-1], part[2*MAX_N-1];
int m;
   
void init(int n){
    m=1;
    while(m<n) m*=2;
    for(int i=0; i<2*m-1; i++){
        mn[i]=INF;
        part[i]=INF;
    }
}
  
void eval(int k, int l, int r){
    if(part[k]<INF){
        mn[k]=min(mn[k], part[k]);
        if(k<m-1){
            part[2*k+1]=min(part[k], part[2*k+1]);
            part[2*k+2]=min(part[k], part[2*k+2]);
        }
        part[k]=INF;
    }
}
    
void update(int a, int b, int x, int k, int l, int r){
	eval(k, l, r);
	if(r<=a || b<=l) return;
    if(a<=l && r<=b){
        part[k]=min(x, part[k]);
        eval(k, l, r);
    }else{
        update(a, b, x, k*2+1, l, (l+r)/2);
        update(a, b, x, k*2+2, (l+r)/2, r);
        mn[k]=min(mn[2*k+1], mn[2*k+2]);
    }
}
    
int find(int a, int b, int k, int l, int r){
	eval(k, l, r);
    if(b<=l || r<=a){
        return INF;
    }
    if(a<=l && r<=b){
        return mn[k];
    }else{
        return min(find(a, b, 2*k+1, l, (l+r)/2), find(a, b, 2*k+2, (l+r)/2, r));
    }
}

int main()
{
	int n;
	cin>>n;
	init(n+1);
	for(int i=0; i<n; i++){
		int a;
		cin>>a;
		int i1=1, i2=n+1;
		while(i1!=i2){
			int i0=(i1+i2)/2;
			if(find(0, i0+1, 0, 0, m)>=a-i) i1=i0+1;
			else i2=i0;
		}
		update(i1-1, n+1, a-i-1, 0, 0, m);
	}
	int i1=1, i2=n+1;
	while(i1!=i2){
		int i0=(i1+i2)/2;
		if(find(0, i0+1, 0, 0, m)>=INF) i1=i0+1;
		else i2=i0;
	}
	cout<<i1-1<<endl;
	return 0;
}