#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define REP(i, n)           for(int(i)=0;(i)<(n);++(i))
const int MOD = 1000000007;

int N, a[51], b[51];
bool dfs(int i){
    if(i == N) return true;

    if(b[i] >= 0){
        int j = b[i];
        if(b[j] >= 0){
            if(a[i] != b[j]) return false;
            if(dfs(i+1)) return true;
        } else {
            b[j] = a[i];
            if(dfs(i+1)) return true;
            b[j] = -1;
        }
    } else {
        REP(j,N){
            b[i] = j;
            if(b[j] >= 0){
                if(a[i] != b[j]) continue;
                if(dfs(i+1)) return true;
            } else {
                b[j] = a[i];
                if(dfs(i+1)) return true;
                b[j] = -1;
            }
        }
        b[i] = -1;
    }
    return false;
}

int main(){
    cin >> N;
    REP(i,N){
        cin >> a[i]; a[i]--;
        b[i] = -1;
    }

    cout << (dfs(0)?"Yes":"No") << endl;
    return 0;
}