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

using ll = long long;
using llu = long long unsigned;

int main() {
    int C, Y;
    cin >> C >> Y;

    if (Y < 100*C) { cout << "can't exchange"; return 0; }

    int tmp = (Y % 500) / 100;
    if (C <= tmp) { cout << "no exchange"; return 0; }

    int ret = ((C - tmp + 4)/5)*5 + tmp;
    cout << ret << endl; return 0;
}