Codeforces 844 A Diversity

#include <iostream>
#include <cstring>
#include <string>
#include <queue>
#include <vector>
#include <map>
#include <set>
#include <stack>
#include <cmath>
#include <cstdio>
#include <algorithm>
#include <iomanip>
#define N 10100
#define M 500010
#define LL __int64
#define inf 0x3f3f3f3f3f3f3f3f
#define lson l,mid,ans<<1
#define rson mid+1,r,ans<<1|1
#define getMid (l+r)>>1
#define movel ans<<1
#define mover ans<<1|1
using namespace std;
const LL mod = 1e9 + 7;
set<char>s;
int main() {
cin.sync_with_stdio(false);
string str;
int n;
while (cin >> str) {
cin >> n;
s.clear();
if (str.length() < n) {
cout << "impossible" << endl;
}
else {
for (int i = 0; i < str.length(); i++) {
s.insert(str[i]);
}
if (n <= s.size()) {
cout << "0" << endl;
}
else {
cout << n - s.size() << endl;
}
}
}
return 0;
}