#include <iostream>
#include <map>
#include <cmath>
 
 
int firstDigit(int number);
int sumDigits(int number);
int removeFirstDigit(int number);
 
int main() {
 
	std::map<int, int> pairs;
 
    for(int i = 1000; i <= 9999; ++i) {
        int stripped = firstDigit(i);
        int threeDigit = removeFirstDigit(i);
        int firstDigit3 = firstDigit(threeDigit);
        if ( threeDigit == sumDigits(stripped) && firstDigit3 > 0 ) {
          pairs[i] = threeDigit;
        }
    }
 
    for(auto i : pairs) {
        int key = i.first;
        int value = i.second;
        std::cout << "1st digit of " << key << " is equal to digit summation of " << value << std::endl;
    }
 
    std::cout << "There are " << pairs.size() << " combination!" << std::endl;
 
    return 0;
}
 
int sumDigits(int number) {
    int sum = 0;
    while ( number > 0 ) {
        sum += number % 10;
        number /= 10;
    }
    return sum;
}
 
int removeFirstDigit(int number) {
    return number % static_cast<int>(std::pow(10, static_cast<size_t>(std::log10(number))));
}
 
int firstDigit(int number) {                         
	while(number > 9)
    	number /= 10;
    return number;
}

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