python/TangDou/LuoGuBook/20_1_8.cpp

#include <bits/stdc++.h>

using namespace std;
int cnt;

int main() {
    for (int i = 10000; i <= 19999; i++) {
        int n = i;
        while (n) {
            if (n % 10 == 4) {
                cnt++;
                break;
            }
            n /= 10;
        }
    }
    cout << cnt << endl;
    return 0;
}