#include <bits/stdc++.h>   using namespace std;   int n, x, r[100000], pos[100000], flag = 1, flag2;   pair<int, int> a[100000];   vector<int> ans, ans2;   int l(int n) {     if (!n) return 1;     if (n % 10 == 4 || n % 10 == 7) return l(n / 10);     return 0;   }   int main() {     scanf( %d , &n);     for (int i = 0; i < n; i++) {       scanf( %d , &x);       a[i] = pair<int, int>(x, i);     }     sort(a, a + n);     for (int i = 0; i < n; i++) {       r[a[i].second] = i;       pos[i] = a[i].second;       if (a[i].second != i) flag = 0;       if (!flag2 && l(a[i].first)) x = i, flag2 = 1;     }     if (flag) {       cout << 0 << endl;       return 0;     }     if (!flag2) {       cout << -1 << endl;       return 0;     }     for (int i = 0; i < n; i++)       if (i != x) {         if (pos[i] == i) continue;         if (pos[x] != i) {           ans.push_back(pos[x]), ans2.push_back(i);           int temp = pos[x];           swap(pos[x], pos[r[i]]);           swap(r[temp], r[i]);         }         ans.push_back(pos[x]), ans2.push_back(pos[i]);         swap(pos[x], pos[i]);         swap(r[pos[x]], r[pos[i]]);       }     cout << ans.size() << endl;     for (int i = 0; i < ans.size(); i++)       cout << ans[i] + 1 <<     << ans2[i] + 1 << endl;   }   