#include <bits/stdc++.h>
using namespace std;
int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
int n, m;
int lst, ans;
int main() {
  n = read();
  lst = n + 1;
  for (int i = 1; i <= n; i++) {
    if (i == read()) {
      if (lst == i - 1) {
        lst = n + 1;
        ans--;
      } else {
        ans++;
        lst = i;
      }
    }
  }
  printf("%d\n", ans);
}
