#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vd = vector<double>;
using vvd = vector<vd>;
using vs = vector<string>;
using vb = vector<bool>;
using vvb = vector<vb>;
const int INF = (1 << 30) - 1;
const ll INF64 = ((ll)1 << 62) - 1;
const double PI = 3.1415926535897932384626433832795;
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, 1, 0, -1};
int gcd(int x, int y) { return y ? gcd(y, x % y) : x; }
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
int lcm(int x, int y) { return x / gcd(x, y) * y; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vi p(n);
  vb state(n);
  for (int i = 0; i < n; i++) {
    cin >> p[i];
    state[i] = p[i] == i + 1;
  }
  int res = 0;
  for (int i = 0; i < n - 1; i++) {
    if (state[i]) {
      state[i] = state[i + 1] = false;
      res++;
    }
  }
  cout << res << endl;
  return 0;
}
