当你不想写太多时

for each 太好用了（

#include <bits/stdc++.h>
using namespace std;
int l, r, sum;
int f (int x) {
    int cnt = 0;
    string s = to_string(x);
    for (char c : s) if (c == '2') cnt++;
    return cnt;
}
int main() {
    cin >> l >> r;
    for (int i = l; i <= r; i++) sum += f(i);
    cout << sum << endl;
    return 0;
}