#include <bits/stdc++.h>
using ll = long long;
// #define int ll
#define FOR(i, a, b) for (int i = (a); i < int(b); ++i)
#define RFOR(i, a, b) for (int i = (b)-1; i >= int(a); --i)
#define rep(i, n) FOR(i, 0, n)
#define rep1(i, n) FOR(i, 1, int(n) + 1)
#define rrep(i, n) RFOR(i, 0, n)
#define rrep1(i, n) RFOR(i, 1, int(n) + 1)
#define all(c) begin(c), end(c)
// const int MOD = 1000000007;

template <typename T>
void __print__(std::ostream &os, const char *, const char *tail, const T &fst) {
    os << fst << tail;
}
template <typename Fst, typename... Rst>
void __print__(std::ostream &os, const char *del, const char *tail, const Fst &fst,
               const Rst &... rst) {
    os << fst << del;
    __print__(os, del, tail, rst...);
}

#ifdef LOCAL
#define dump(...)                                         \
    do {                                                  \
        std::ostringstream os;                            \
        os << __LINE__ << ":\t" << #__VA_ARGS__ << " = "; \
        __print__(os, ", ", "\n", __VA_ARGS__);           \
        std::cerr << os.str();                            \
    } while (0)
#else
#define dump(...)
#endif

template <typename Fst, typename... Rst>
void println(const Fst &fst, const Rst &... rst) {
    __print__(std::cout, "\n", "\n", fst, rst...);
}
template <typename Fst, typename... Rst>
void print(const Fst &fst, const Rst &... rst) {
    __print__(std::cout, " ", "\n", fst, rst...);
}

template <typename iter>
void println_(iter bgn, iter end) {
    while (bgn != end) println(*bgn++);
}

template <typename iter>
void print_(iter bgn, iter end) {
    while (bgn != end) {
        std::cout << *bgn++;
        std::cout << (bgn == end ? "\n" : " ");
    }
}

// int _ = (std::cout.precision(10), std::cout.setf(std::ios::fixed), std::cin.tie(0),
//          std::ios::sync_with_stdio(0), 0);

template <typename T>
std::vector<T> ndarray(int n, T v) {
    return std::vector<T>(n, v);
}
template <typename... Args>
auto ndarray(int n, Args... args) {
    auto val = ndarray(args...);
    return std::vector<decltype(val)>(n, move(val));
}

template <typename T>
bool umax(T &a, const T &b) {
    return a < b ? a = b, true : false;
}

template <typename T>
bool umin(T &a, const T &b) {
    return a > b ? a = b, true : false;
}

using namespace std;

int solve(vector<int> p) {
    const auto org = p;
    int n = p.size();
    // int ans = 1e9;
    // {
    //     int cnt = 0;
    //     rep(i, n - 1) {
    //         if (p[i] == i || p[i + 1] == i + 1) {
    //             swap(p[i], p[i + 1]);
    //             ++cnt;
    //         }
    //     }
    //     if (p[n - 1] != n - 1) {
    //         ans = min(ans, cnt);
    //     }
    // }
    // {
    //     int cnt = 0;
    //     p = org;
    //     for (int i = n - 1; i >= 1; --i) {
    //         if (p[i] == i || p[i - 1] == i - 1) {
    //             swap(p[i], p[i - 1]);
    //             ++cnt;
    //         }
    //     }
    //     if (p[0] != 0) {
    //         ans = min(ans, cnt);
    //     }
    // }
    // assert(ans != 1e9);

    int ans = 0;
    rep(i, n) {
        if (p[i] != i) continue;
        int j = i;
        while (j < n && p[j] == j) ++j;
        ans += (j - i + 1) / 2;
        i = j;
    }
    return ans;
}

int main() {
    int n;
    while (cin >> n) {
        vector<int> p(n);
        rep(i, n) cin >> p[i], --p[i];
        print(solve(p));
    }

    // {
    //     int n = 4;
    //     vector<int> p(n);
    //     iota(all(p), 0);
    //     do {
    //         cout << "a" << endl;
    //         print_(all(p));
    //         dump(solve(p));
    //     } while (next_permutation(all(p)));
    // }

}


Measured memory_bytes: 1408