#include <iostream>

using namespace std;

int main() {
	int a;
	cin >> a;
	int b[a];
	int c[a];
	int ans = 0;
	for (int i = 0;i < a;i++) {
		cin >> b[i];
		c[i] = b[i];
	}
	for (int i = 0;i < a - 1;i++) {
		if (b[i] == i + 1) {
			ans++;
			b[i + 1] = b[i];
		}
	}
	if (b[a - 1]== a) {ans++;}
	cout << ans;
}