#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  vector<int> p(N + 2);
  for (int i = 1; i <= N; i++) cin >> p.at(i);
  p.at(0) = -1;
  p.at(N + 1) = -1;
  int sum = 0;
  for (int i = 1; i <= N + 1; i++) {
    if (p.at(i) == i)
      sum++;
    else {
      p.push_back(sum);
      sum = 0;
    }
  }
  int ans = 0;
  for (auto x : p) {
    ans += (1 + x) / 2;
  }
  cout << ans << endl;
  return 0;
}
