#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
using ll=long long;
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
template<class T> bool chmax(T &a, T b){if (a < b){a = b;return true;} else return false;}
template<class T> bool chmin(T &a, T b){if (a > b){a = b;return true;} else return false;}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int C,Y;
    cin>>C>>Y;
    string S=to_string(Y);
    int L=S.size();
    if(Y<100){
        if(C>0)cout<<"can't exchange\n";
        else cout<<"no exchange\n";
    }else if(Y<C*100){
        cout<<"can't exchange\n";
    }else{
        int H=(S[L-3]<='4'?S[L-3]-'0':S[L-3]-'5');
        if(C<=H){
            cout<<"no exchange\n";
        }else{
            cout<<5*((C-H+4)/5)+H<<"\n";
        }
    }
}