#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
long long modpow(long long a, long long n, long long m) {
  long long ans = 1;
  while (n) {
    if (n & 1) {
      ans = (ans * a) % m;
    }
    a = (a * a) % m;
    n >>= 1;
  }
  return ans;
}
long long combi(long long n, long long a) {
  long long ans = 1, ans1 = 1;
  for (long long i = n - a + 1; i <= n; i++) {
    ans *= i % MOD;
    ans %= MOD;
  }
  for (long long i = 2; i <= a; i++) ans1 = (ans1 * i) % MOD;
  ans1 = modpow(ans1, MOD - 2, MOD);
  return ((ans % MOD) * ans1) % MOD;
}
template <typename T>
bool next_combination(const T first, const T last, int k) {
  const T subset = first + k;
  if (first == last || first == subset || last == subset) {
    return false;
  }
  T src = subset;
  while (first != src) {
    src--;
    if (*src < *(last - 1)) {
      T dest = subset;
      while (*src >= *dest) {
        dest++;
      }
      iter_swap(src, dest);
      rotate(src + 1, dest + 1, last);
      rotate(subset, subset + (last - dest) - 1, last);
      return true;
    }
  }
  rotate(first, subset, last);
  return false;
}
void dfs(string s, char mx, long long N) {
  if (s.length() == (size_t)N) {
    cout << s.c_str() << endl;
  } else {
    for (char c = 'a'; c <= mx; c++) {
      dfs(s + c, ((c == mx) ? (char)(mx) : mx), N);
    }
  }
}
int bfs(int a, int b, int h, int w, char tmp[][101]) {
  int ans[101][101] = {0};
  char c[101][101] = {0};
  queue<pair<int, int>> s;
  for (int i = 0; i <= h; i++) {
    for (int j = 0; j <= w; j++) {
      c[i][j] = tmp[i][j];
    }
  }
  s.push(make_pair(a, b));
  while (s.size() > 0) {
    pair<int, int> tmp = s.front();
    s.pop();
    c[tmp.first][tmp.second] = '#';
    for (int i = -1; i <= 1; i++) {
      for (int j = -1; j <= 1; j++) {
        if (tmp.first + i < 1 || tmp.first + i > h) {
          continue;
        }
        if (tmp.second + j < 1 || tmp.second + j > w) {
          continue;
        }
        if (i != 0 && j != 0) {
          continue;
        }
        if (i == 0 && j == 0) {
          continue;
        }
        if (c[tmp.first + i][tmp.second + j] == '#') {
          continue;
        }
        c[tmp.first + i][tmp.second + j] = '#';
        if (ans[tmp.first + i][tmp.second + j] == 0) {
          ans[tmp.first + i][tmp.second + j] = ans[tmp.first][tmp.second] + 1;
        } else {
          ans[tmp.first + i][tmp.second + j] =
              min(ans[tmp.first + i][tmp.second + j],
                  ans[tmp.first][tmp.second] + 1);
        }
        s.push(make_pair(tmp.first + i, tmp.second + j));
      }
    }
  }
  int asd = 0;
  for (int i = 1; i <= h; i++) {
    for (int j = 1; j <= w; j++) {
      asd = max(a, ans[i][j]);
    }
  }
  return asd;
}
long long modfactorial(long long a) {
  if (a == 1) return 1;
  return (a % MOD) * (modfactorial(a - 1) % MOD);
}
long long gcd(long long a, long long b) {
  if (a % b == 0) {
    return (b);
  } else {
    return (gcd(b, a % b));
  }
}
long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }
int main() {
  long long n;
  cin >> n;
  vector<long long> v(n);
  for (int i = 0; i < n; i++) {
    cin >> v[i];
  }
  long long ans = 0;
  for (int i = 0; i <= n / 2; i++) {
    if (2 * i + 1 == v[2 * i] || 2 * i + 2 == v[2 * i + 1]) {
      ans++;
    }
  }
  if (n % 2) {
    if (v[n - 1] == n) {
      ans++;
    }
  }
  cout << ans << endl;
  return 0;
}
