#include <bits/stdc++.h>
using namespace std;
const int inf = (int)1e9;
const long long linf = (long long)1e18;
const int mod = (int)(1e9 + 7);
const int dx[] = {0, 1, 0, -1};
const int dy[] = {1, 0, -1, 0};
const int ddx[] = {0, 1, 1, 1, 0, -1, -1, -1};
const int ddy[] = {1, 1, 0, -1, -1, -1, 0, 1};
struct oreno_initializer {
  oreno_initializer() {
    cin.tie(0);
    ios::sync_with_stdio(0);
  }
} oreno_initializer;
int n, p[111111];
int main() {
  cin >> n;
  for (int(i) = 0; (i) < (int)(n); (i)++) cin >> p[i];
  int ans = 0;
  for (int(i) = 0; (i) < (int)(n - 1); (i)++) {
    if (p[i] == i + 1) {
      swap(p[i], p[i + 1]);
      ans++;
    }
  }
  cout << ans << "\n";
}
