#include<bits/stdc++.h>
using namespace std;
const int maxn = 8005;
const int S = 600;
int n,p[maxn],s[maxn][4];
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);cout.tie(NULL);
cin >> n;
for(int i=1;i<=n;i++){
cin >> p[i];
s[i][0]=min(n-i,n-p[i]);
s[i][1]=min(i-1,n-p[i]);
s[i][2]=min(n-i,p[i]-1);
s[i][3]=min(i-1,p[i]-1);
}
int k=0;
while(true){
vector<array<int,3>> sq;
for(int i=1;i<=n;i++) for(int j=0;j<=3;j++) if(s[i][j]!=-1) sq.push_back({-s[i][j],i,j}),s[i][j]=-1;
if(sq.empty()) break;
sort(sq.begin(),sq.end());
if((int)sq.size()>S) sq.erase(sq.begin()+S,sq.end());
for(auto [d,i,j]:sq){
d=-d;
int x=-1,y=-1,u=-1,v=-1;
if(j&1) x=i-d,u=i-1;
else x=i+1,u=i+d;
if(j&2) y=p[i]-d,v=p[i]-1;
else y=p[i]+1,v=p[i]+d;
for(int t=x;t<=u;t++){
if(p[t]<y || p[t]>v) continue;
s[t][0]=max(s[t][0],min(u-t,v-p[t]));
s[t][1]=max(s[t][1],min(t-x,v-p[t]));
s[t][2]=max(s[t][2],min(u-t,p[t]-y));
s[t][3]=max(s[t][3],min(t-x,p[t]-y));
}
}
k++;
}
cout << n-k << '\n';
}
