#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-10;
const double PI = acos(-1.0);
template <typename T>
void vprint(vector<T> v) {
  for (auto x : v) {
    cerr << x << " ";
  }
  cerr << endl;
}
template <typename T>
void vvprint(vector<vector<T>> vv) {
  for (int i = (0); i < (int((vv).size())); ++i) {
    for (int j = (0); j < (int((vv[i]).size())); ++j) {
      cerr << vv[i][j] << " ";
    }
    cerr << endl;
  }
}
template <typename T1, typename T2>
void vpprint(vector<pair<T1, T2>> vp) {
  for (int i = (0); i < (int((vp).size())); ++i) {
    cerr << vp[i].first << ", " << vp[i].second << endl;
  }
}
template <typename T1, typename T2>
void mprint(map<T1, T2> m) {
  for (auto x : m) cerr << x.first << ", " << x.second << endl;
}
template <typename Iterator>
inline bool next_combination(const Iterator first, Iterator k,
                             const Iterator last) {
  if ((first == last) || (first == k) || (last == k)) return false;
  Iterator itr1 = first;
  Iterator itr2 = last;
  ++itr1;
  if (last == itr1) return false;
  itr1 = last;
  --itr1;
  itr1 = k;
  --itr2;
  while (first != itr1) {
    if (*--itr1 < *itr2) {
      Iterator j = k;
      while (!(*itr1 < *j)) ++j;
      iter_swap(itr1, j);
      ++itr1;
      ++j;
      itr2 = k;
      rotate(itr1, j, last);
      while (last != j) {
        ++j;
        ++itr2;
      }
      rotate(k, itr2, last);
      return true;
    }
  }
  rotate(first, k, last);
  return false;
}
inline double get_time_sec(void) {
  return static_cast<double>(chrono::duration_cast<chrono::nanoseconds>(
                                 chrono::steady_clock::now().time_since_epoch())
                                 .count()) /
         1000000000;
}
template <typename T>
T gcd(T a, T b) {
  if (a > b) swap(a, b);
  if (a == 0)
    return b;
  else
    return gcd(b % a, a);
}
template <typename T>
T lcm(T a, T b) {
  return (a / gcd(a, b)) * b;
}
template <typename T>
map<T, T> prime_list(T n) {
  map<T, T> ret;
  for (T i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      ret[i] = 0;
      while (n % i == 0) {
        n /= i;
        ret[i]++;
      }
    }
  }
  if (n != 1) ret[n]++;
  return ret;
}
long long mypow(long long a, long long n) {
  if (n == 0) return 1;
  if (n == 1) return a % 1000000007;
  if (n % 2 == 1) return (a * mypow(a, n - 1)) % 1000000007;
  long long t = mypow(a, n / 2);
  return (t * t) % 1000000007;
}
vector<long long> _fact, _inv;
bool _fact_flg = true;
void _fact_init() {
  _fact = vector<long long>(100010);
  _inv = vector<long long>(100010);
  _fact[0] = 1;
  for (int i = (1); i < (100010); ++i)
    _fact[i] = (_fact[i - 1] * i) % 1000000007;
  _inv[100010 - 1] = mypow(_fact[100010 - 1], 1000000007 - 2);
  for (int i = 100010 - 2; i >= 0; i--) {
    _inv[i] = ((i + 1) * _inv[i + 1]) % 1000000007;
  }
}
long long mycomb(long long n, long long k) {
  if (_fact_flg) {
    _fact_flg = false;
    _fact_init();
  }
  if (n < k) return 0;
  return (((_fact[n] * _inv[k]) % 1000000007) * _inv[n - k]) % 1000000007;
}
vector<long long> par, rnk, sz;
int root(int x) {
  if (par[x] == x)
    return x;
  else
    return par[x] = root(par[x]);
}
bool same(int x, int y) {
  x = root(x);
  y = root(y);
  return x == y;
}
void unite(int x, int y) {
  x = root(x);
  y = root(y);
  if (rnk[x] < rnk[y]) {
    par[x] = y;
  } else {
    par[y] = x;
    if (rnk[x] == rnk[y]) rnk[x]++;
  }
}
int main(void) {
  long long n;
  cin >> n;
  vector<long long> a(n);
  for (int i = (0); i < (n); ++i) {
    cin >> a[i];
  }
  long long cnt = 0;
  for (int i = (0); i < (n - 1); ++i) {
    if (a[i] == i + 1) {
      cnt++;
      swap(a[i], a[i + 1]);
    }
  }
  cout << cnt << endl;
}
