#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> P(N);
  for (ll i = 0; i < (ll)(N); i++) cin >> P[i];
  ll ans = 0;
  for (ll i = 0; i < (ll)(N - 1); i++) {
    ll p = P[i];
    if (p == i + 1) {
      ll tmp = P[i + 1];
      P[i + 1] = p;
      P[i] = tmp;
      ans++;
    }
  }
  cout << ans << '\n';
}
int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  Main();
  return 0;
}
