#include <iostream>
#include <vector>

using namespace std;

int main()
{
	int n;
	cin >> n;

	vector<int> a(n);
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	a.emplace_back(0);

	int ans = 0;
	for (int i = 0; i < n; i++) {
		if (a[i] == i + 1) {
			ans++;
			swap(a[i], a[i + 1]);
		}
	}
	cout << ans << endl;

	return 0;
}