#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
template <class S, class T>
ostream& operator<<(ostream& os, const pair<S, T> v) {
  os << "(" << v.first << ", " << v.second << ")";
  return os;
}
template <class T>
ostream& operator<<(ostream& os, const vector<T> v) {
  for (int i = 0; i < (int)v.size(); i++) {
    if (i > 0) {
      os << " ";
    }
    os << v[i];
  }
  return os;
}
template <class T>
ostream& operator<<(ostream& os, const vector<vector<T>> v) {
  for (int i = 0; i < (int)v.size(); i++) {
    if (i > 0) {
      os << endl;
    }
    os << v[i];
  }
  return os;
}
template <class T>
bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T& a, const T& b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
string num2bit(long long num, long long len) {
  string bit = "";
  for (int i = 0; i < (int)len; ++i) {
    bit += char('0' + (num >> i & 1));
  }
  return bit;
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  long long N;
  cin >> N;
  vector<long long> p(N);
  for (int i = 0; i < (int)N; ++i) cin >> p[i];
  long long res = 0;
  for (int i = 0; i < (int)N - 1; ++i) {
    if (p[i] == i + 1) {
      swap(p[i], p[i + 1]);
      res++;
    }
  }
  cout << res << endl;
  return 0;
}
