#ifdef PROBLEM_ABC120_C  //コンテスト番号と問題番号

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

int main() {
	char S[100001];
	cin >> S;

	int cnt_0 = 0;
	int cnt_1 = 0;
	size_t len = strlen(S);

	for (int i = 0; i < len; i++) {
		if (S[i] == '0')
			cnt_0++;
		else if (S[i] == '1')
			cnt_1++;
	}

	cout << 2 * min(cnt_0, cnt_1) << endl;

	return 0;
}
