#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;
const ll MOD = 1000000007;
const ll INF = 10000000000000000;
vector<int> x4 = {0, 1, 0, -1}, x8 = {0, 1, 1, 1, 0, -1, -1, -1};
vector<int> y4 = {1, 0, -1, 0}, y8 = {1, 1, 0, -1, -1, -1, 0, 1};
template <class T>
struct edge {
  int from, to;
  T cost;
};
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline T powerM(T a, T b) {
  if (b == 0) return 1;
  T tmp = powerM(a, b / 2);
  if (b % 2 == 0)
    return tmp * tmp % MOD;
  else
    return tmp * tmp % MOD * a % MOD;
}
template <class T>
inline T power(T a, T b, T m) {
  if (b == 0) return 1;
  T tmp = power(a, b / 2, m);
  if (b % 2 == 0)
    return tmp * tmp % m;
  else
    return tmp * tmp % m * a % m;
}
template <class T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <class T>
inline T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
template <class T>
inline T extgcd(T a, T b, T& x, T& y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  }
  T d = extgcd(b, a % b, y, x);
  y -= a / b * x;
  return d;
}
void hey() { cout << "hey" << endl; }
int main() {
  int n;
  cin >> n;
  vector<int> P(n);
  for (int i = 0; i < (int)(n); i++) cin >> P[i];
  bool sa = false;
  int res = 0;
  for (int i = 0; i < (int)(n); i++) {
    if (i == P[i]) {
      if (sa) {
        res++;
        sa = false;
      } else
        sa = true;
    } else {
      if (sa) {
        res++;
        sa = false;
      }
    }
  }
  if (sa) res++;
  cout << res << endl;
}
