844A. Diversity

/***

            Bismillahir Rahmanir Rahim
            Read the name of Allah, who created you!!!
            Author : Shah Newaj Rabbi Shishir
            Department of CSE, City University, Bangladesh.

***/

#include <bits/stdc++.h>
using namespace std;

#define sf scanf
#define pf printf
#define scase sf ("%d",&tc)
#define sn sf ("%d",&n)
#define whilecase while (tc--)
#define eof while (cin >> n)
#define forloop for (pos=1; pos<=tc; pos++)
#define arrayloop (i=0; i<n; i++)
#define cinstr cin >> str
#define getstr getline (cin,str)
#define pcase pf ("Case %d: ",pos)
#define pb push_back
#define in insert
#define llu unsigned long long
#define lld long long
#define U unsigned int

int main (void)
{
    /*freopen ("input.txt","r",stdin);
    freopen ("output.txt","w",stdout);*/
    string str;
    int k,len,i;

    while (cin >> str >> k)
    {
        len = str.size();

        if (len < k)
            cout << "impossible\n";
        else
        {
            set <char> myset;

            for (i=0; i<len; i++)
                myset.in(str[i]);

            len = myset.size();

            if (len >= k)
                cout << 0 << endl;
            else
                cout << k-myset.size() << endl;
        }
    }

    return 0;
}