#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
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 (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
void solve(long long N, std::vector<long long> p) {
  long long ans = 0;
  for (long long i = 0, i_len = (N - 1); i < i_len; ++i) {
    if (p[i] == i + 1) {
      ans++;
      swap(p[i], p[i + 1]);
    }
  }
  cout << ans << endl;
  ;
}
int main() {
  long long N;
  scanf("%lld", &N);
  std::vector<long long> p(N);
  for (int i = 0; i < N; i++) {
    scanf("%lld", &p[i]);
  }
  solve(N, std::move(p));
  return 0;
}
