#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
using Graph = vector<vector<ll>>;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const int ddx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const int ddy[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll MOD = 1000000007;
const ll INF = 1000000000000000000L;
void Main() {
  ll N;
  cin >> N;
  vector<ll> ng;
  vector<ll> p(N);
  for (ll i = 0; i < (ll)(N); i++) {
    cin >> p.at(i);
    if (p.at(i) == (i + 1)) {
      ng.push_back(i);
    }
  }
  ll i = 0;
  ll ans = 0;
  while (i < ng.size()) {
    if ((ng.at(i) + 1) == ng.at(i + 1)) {
      i += 2;
    } else {
      i += 1;
    }
    ans++;
  }
  cout << ans << endl;
}
int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  Main();
  return 0;
}
