#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
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;
  vector<int> p(n);
  for (int i = 0; i < n; i++) {
    cin >> p[i];
  }
  int ans = 0;
  for (int i = 0; i < n - 1; i++) {
    if (p[i] == (i + 1)) {
      swap(p[i], p[i + 1]);
      ans++;
    }
  }
  cout << ans << endl;
  return 0;
}
