#include <bits/stdc++.h>   using namespace std;   const int maxn = 100000;   struct node {     int n;     int i;     node(int n, int i) : n(n), i(i) {}     node() {}     bool operator<(const node& n1) const { return n < n1.n; }     bool is_happy() const {       int a = n;       while (a) {         if (a % 10 != 7 && a % 10 != 4) return false;         a /= 10;       }       return true;     }   };   node nodes[maxn];   int rev[maxn];   inline void sw(int i, int j, vector<pair<int, int> >& ans) {     ans.push_back(pair<int, int>(nodes[i].i, nodes[j].i));     swap(nodes[i].i, nodes[j].i);     rev[nodes[i].i] = i;     rev[nodes[j].i] = j;   }   int main() {     int n;     scanf( %d , &n);     bool sorted = true;     int x = -1, prev;     for (int i = 0; i < n; i++) {       prev = x;       scanf( %d , &x);       if (prev > x) sorted = false;       nodes[i] = node(x, i);     }     if (sorted) {       printf( 0 n );       return 0;     }     sort(nodes, nodes + n);     for (int i = 0; i < n; i++) {       rev[nodes[i].i] = i;     }     int happy = -1;     for (int i = 0; i < n; i++) {       if (nodes[i].is_happy()) {         happy = i;         break;       }     }     if (happy == -1) {       printf( -1 n );       return 0;     }     vector<pair<int, int> > ans;     for (int i = 0; i < n; i++)       if (i != happy && nodes[i].i != i) {         if (rev[i] != happy) sw(rev[i], happy, ans);         sw(happy, i, ans);       }     printf( %d n , ans.size());     for (int i = 0; i < ans.size(); i++) {       printf( %d %d n , ans[i].first + 1, ans[i].second + 1);     }     return 0;   }   