#include <bits/stdc++.h>
using namespace std;

#define int long long
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define repo(i,o,n) for(int i=o;i<n;i++)
#define repm(i,n) for(int i=n-1;i>=0;i--)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define sperase(v,n) (v).erase(remove(all(v), n), (v).end());
#define vdelete(v) (v).erase(unique(all(v)), (v).end());
#define pb(n) push_back(n);

int n,p,memo,ans;

main() {
    cin >> n;
    rep1(i,n) {
        cin >> p;
        if (p == i) {
            if (memo) memo = 0,ans++;
            else memo = 1;
        } else if (memo) memo = 0,ans++;
    }
    cout << ans << endl;
}

