#include <string.h>
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int c,y;
    cin >> c >> y;
    vector<int>a = {10000,5000,2000,1000,500,100};
    int sum = 0,res = 0;
    for(int i = 0; i < a.size(); i++) {
        int now = y/a[i];
        y %= a[i];
        if(a[i] >= 500) sum += now*a[i]/500;
        else {
            res = now;
        }
    }
    if(res >= c) {
        cout << "no exchange" << "\n";
    }
    else {
        if(sum*5+res < c) {
            cout << "can't exchange" << "\n";
        }
        else {
            c -= res;
            cout << (5-c%5)%5 << "\n";
        }
    }
}