NearlyLuckyNumber.cpp

/* Petya loves lucky numbers. We all know that lucky numbers are the positive integers 
whose decimal representations contain only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.

Unfortunately, not all numbers are lucky. Petya calls a number nearly lucky if the number of lucky digits in it is a lucky number. 
He wonders whether number n is a nearly lucky number.

Input
The only line contains an integer n (1 ≤ n ≤ 1018).

Please do not use the %lld specificator to read or write 64-bit numbers in С++. It is preferred to use the cin, 
cout streams or the %I64d specificator.

Output
Print on the single line "YES" if n is a nearly lucky number. Otherwise, print "NO" (without the quotes). */

#include <iostream>

using namespace std;

int main()
{
    string num;
    int sum;
    cin >> num;
    for(int i = 0; i < num.length(); i++){
        if(num[i] == '4' || num[i] == '7'){
            sum++;
        }
    }

    if(sum == 7 || sum == 4){
        cout << "YES";
    }else{
        cout << "NO";
    }

    return 0;
}