K - Nearly Lucky Number

#include <bits/stdc++.h>
using namespace std;

bool lucky(int n){
    if(!n)
        return false;
    while(n){
        int d = n % 10;
        if((d != 4) && (d != 7))
            return false;
        n /= 10;
    }
    return true;
}

signed main()
{
    string s;
    cin >> s;
    int cnt = 0;
    for(int i = 0; i < s.size(); i++){
        if((s[i] == '4') || (s[i] == '7'))
            cnt++;
    }
    cout << ((lucky(cnt)) ? "YES" : "NO") << endl;
}