#include <bits/stdc++.h>

using namespace std;

const int len = 100010;
int n,a[len];

int main() {
	ios :: sync_with_stdio(0);
	cin >> n;
	for (int i=1;i<=n;i++) {
		cin >> a[i];
	}
	int sum = 0;
	for (int i=1;i<=n;i++) {
		if (a[i] == i) {
			swap(a[i],a[i+1]);
			sum++;
		}
	}
	cout << sum << endl;
	return 0;
}