#include<bits/stdc++.h>
#include<atcoder/all>
#define chmin(x,y) (x) = min((x),(y))
#define chmax(x,y) (x) = max((x),(y))
#define ld long double
using namespace std;
using namespace atcoder;
using ll = long long;
const ll mod = 998244353;
using mint = modint998244353;
//using Graph = vector<vector<pair<int,int>>>;
using Graph = vector<vector<int>>;
const vector<int> dx = {1,0,-1,0}, dy = {0,1,0,-1};

ll sqrtll(ll x) {
  assert(x >= 0);
  ll hi(x), lo(0);
  while (hi != lo) {
    ll y = (hi + lo + 1) / 2;
    if (y <= x/y) lo = y;
    else hi = y - 1;
  }
  return lo;
}


int main(){
  // input 
  int C,Y; cin >> C >> Y;
  int bill = Y / 500, coin = (Y % 500) / 100;
  if(coin >= C) cout << "no exchange" << endl;
  else if(Y / 100 < C) cout << "can't exchange" << endl;
  else{
    int exc = (C-coin) / 5 + ((C-coin)%5>0);
    int ans = exc * 5 + coin;
    cout << ans << endl;
  }
}