#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using dll = deque<ll>;
using pll = pair<ll, ll>;
using qll = queue<ll>;
using usll = unordered_set<ll>;
using umll = unordered_map<ll, ll>;
using sll = stack<int>;
using vll = vector<ll>;
using vvll = vector<vll>;
constexpr ll kMod = 1e9 + 7;
ll gcd(ll a, ll b) {
  if (a < b) {
    return gcd(b, a);
  }
  ll c = a % b;
  while (c != 0) {
    a = b;
    b = c;
    c = a % b;
  }
  return b;
}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
void init() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}
int main() {
  init();
  ll N;
  cin >> N;
  vll P(N);
  bool all_ok = true;
  for (ll(i) = (0); (i) < (N); (i)++) {
    ll p;
    cin >> p;
    P[i] = p - 1;
    if (P[i] == i) {
      all_ok = false;
    }
  }
  if (all_ok) {
    cout << 0 << endl;
    return 0;
  }
  ll ans = 0;
  for (ll(i) = (0); (i) < (N + 1); (i)++) {
    if (P[i] == i || P[i + 1] == i + 1) {
      ans++;
      ll tmp = P[i];
      P[i] = P[i + 1];
      P[i + 1] = tmp;
    }
  }
  cout << ans << endl;
  return 0;
}
