s,n = input(), int(input())
print("impossible" if n > len(s) else max(0, n - len(set(s))))
#include<bits/stdc++.h>
using namespace std;
int main()
{
string s;
cin>>s;
int k;
cin>>k;
int count=1;
if(s.size()<k)
cout<<"impossible"<<endl;
else
{
sort(s.begin(),s.end());
for(int i=0;i<s.size()-1;i++)
{
if(s[i]!=s[i+1])
count++;
}
if(count>=k)
cout<<0<<endl;
else
cout<<k-count<<endl;
}
}
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