import java.util.*;

class Main
{
	public static void main(String[] args) 
	{
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		int arr[] = new int[n];

		for(int i = 0; i < n; i++)
			arr[i] = sc.nextInt();

		int ans = 0;
		for(int i = 0; i < n; i++)
			if(arr[i] == (i + 1))
			{
				ans++;
				i++;
			}

		System.out.println(ans);
	}
}