#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001



int main(){
	
	ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	
	int C,Y;
	cin>>C>>Y;
	
	long long cur = (Y % 500) / 100;
	long long c500 = Y/500;
	if(cur >= C){
		cout<<"no exchange"<<endl;
	}
	else if(cur + c500*5 < C){
		cout<<"can't exchange"<<endl;
	}
	else{
		while(cur < C){
			cur += 5;
			c500 --;
		}
		cout<<cur<<endl;
	}
	return 0;
}