func_code_string: #include <bits/stdc++.h>   using namespace std;   int res;   int a[100], ins[100];   int IsLucky(int x) {     for (; x; x /= 10)       if (x % 10 != 4 && x % 10 != 7) return 0;     return 1;   }   void Dfs(int now, int lim) {     res++;     if ((long long)now * 10 + 4 <= lim) Dfs(now * 10 + 4, lim);     if ((long long)now * 10 + 7 <= lim) Dfs(now * 10 + 7, lim);   }   int main() {     int n, m, k;     scanf( %d%d , &n, &m);     long long cnt = 1;     for (int i = 1;; i++) {       cnt *= i;       if (cnt >= m) {         k = i;         break;       }     }     if (k > n) {       puts( -1 );       return 0;     }     memset(ins, 0, sizeof(ins));     for (int i = 0; i < k; i++) {       cnt = 1;       for (int j = 1; j < k - i; j++) cnt *= j;       int ret = (m - 1) / cnt;       m -= ret * cnt;       for (int j = 0; j < k; j++)         if (!ins[j]) {           if (ret == 0) {             ins[j] = 1;             a[i] = j;             break;           } else             ret--;         }     }     res = 0;     for (int i = 0; i < k; i++)       if (IsLucky(n - k + 1 + i) && IsLucky(n - k + 1 + a[i])) res++;     if (4 <= n - k) Dfs(4, n - k);     if (7 <= n - k) Dfs(7, n - k);     printf( %d n , res);     return 0;   }   