#include <iostream>using namespace std;int fun(int n, int fromPos, int toPos) { int t, tot; if (n == 0) return 0; for (t = 1; t <= 3; t++) if (t != fromPos && t != toPos) break; tot = 0; tot += fun(n - 1, fromPos, t); tot++; tot += fun(n - 1, t, toPos); return tot;} int main() { int n; cin >> n; cout << fun(n, 1, 3) << endl; return 0;}输入：5 输出：_________