#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  ll c, y;
  cin >> c >> y;
  if (y / 100 < c) {
    cout << "can't exchange" << endl;
  }
  else if ((y % 500) / 100 >= c) {
    cout << "no exchange" << endl;
  }
  else {
    ll rem = c - (y % 500) / 100;
    cout << (5 - rem % 5) % 5 + rem << endl;
  }
  return 0;
}