import java.util.Scanner;
 
class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
 
        int n = in.nextInt();
        int[] p = new int[n];
        for(int i = 0; i < n; i++) {
            p[i] = in.nextInt();
        }
        int ans = 0;
        for(int i = 0; i < n; i++) {
            if(p[i] == i + 1) {
                ans++;
                i++;
            }
        }
        System.out.println(ans);
    }
}