import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = Integer.parseInt(sc.next());
		
		int []p = new int [n];
		
		for(int i = 0 ; i < n ;i++) {
			p[i] = Integer.parseInt(sc.next());
		}
		
		int cnt = 0;
		
		for(int i = 0 ; i < n-1 ;i++) {
			if(p[i] == i+1) {
				int temp = p[i+1];
				cnt++;
				p[i+1] = p[i];
				p[i] = temp;
			}
		}
		
		System.out.println(cnt);
	}

}
