#include <vector>
#include <algorithm>
#include <iostream>
#include <set>
#include <cmath>
#include <map>
#include <random>
#include <cassert>
#include <ctime>
#include <cstdlib>
#include <limits.h>
using namespace std;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int N;
cin >> N;
vector<int64_t> v(N); for (int i = 0; i < N; i++) cin >> v[i];
int sm = 0;
for (int i = 0; i < N; i++) {
if (v[i] == 0) {
continue;
}
int cntr = 1;
for (int j = i + 1; j < N; j++) {
if (v[j] == v[i] - cntr) {
cntr++;
v[j] = 0;
}
}
v[i] = 0;
sm++;
}
cout << sm;
}
