#include <bits/stdc++.h>
using namespace std;
int main() {
int t;
cin>>t;
while(t--)
{
int n,c;
cin>>n>>c;
string s;
cin>>s;
map<char,int> m;
for(auto x : s)
{
m[x]++;
}
int a=m.size();
if(a>c)
{
cout<<0<<"\n";
}
else
{
cout<<c-a<<"\n";
}
}
return 0;
}
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