import java.util.Scanner;


public class Main {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int [] a = new int[n];
        for (int i = 0; i < n; i++) {
            a[i]= in.nextInt();
        }
        int b = 0;
        for (int i = 0; i < n; i++) {
            if (a[i]== i +1){
                b ++;
                i++;
            }
            
        }
        System.out.println(b);

    }
}












