#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> a(n + 1, 0);
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
    }

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

    cout << c << endl;
}
