#include <bits/stdc++.h>
using namespace std;
 
const int MAXN = 8005;
int n, P[MAXN], s[MAXN][4];
 
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
 
    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<4;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());
        int S = 600; // 후보 상한
        if((int)sq.size()>S) sq.erase(sq.begin()+S, sq.end());
 
        for(auto [d,i,j]: sq){
            d = -d;
            int x1, x2, y1, y2;
            if(j&1) x1 = i-d, x2 = i-1;
            else x1 = i+1, x2 = i+d;
            if(j&2) y1 = P[i]-d, y2 = P[i]-1;
            else y1 = P[i]+1, y2 = P[i]+d;
            for(int t = max(1,x1); t <= min(n,x2); t++){
                if(P[t]<y1 || P[t]>y2) continue;
                s[t][0] = max(s[t][0], min(x2-t, y2-P[t]));
                s[t][1] = max(s[t][1], min(t-x1, y2-P[t]));
                s[t][2] = max(s[t][2], min(x2-t, P[t]-y1));
                s[t][3] = max(s[t][3], min(t-x1, P[t]-y1));
            }
        }
        k++;
    }
 
    cout << n-k << '\n';
}

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