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

typedef pair<double, int> PDI;
const int N = 1e6 + 10;
const double eps = 1e-9;

char a[N]; 
int sum[N];

int main()
{
	int T, cnt = 0; scanf("%d", &T);
	while (T -- )
	{
		int n; double f; scanf("%d%lf%s", &n, &f, a + 1);
		for (int i = 1; i <= n; i ++ ) sum[i] = sum[i - 1] + a[i] - '0';
		double mn = 1e9; int res = 1e9;
		set<PDI> ag;
		for (int i = 1; i <= n; i ++ )
		{
			ag.insert({1.0 * sum[i - 1] - f * (i - 1), i});
			auto it = ag.lower_bound({1.0 * sum[i] - f * i, 0});
			if (it != ag.end())
			{
				double v = (*it).first - (1.0 * sum[i] - f * i);
				if (mn - v > eps) mn = v, res = (*it).second;
				else if (fabs(mn - v) < eps) res = min(res, (*it).second);
			}
			if (it != ag.begin())
			{
				it -- ;
				double v = (1.0 * sum[i] - f * i) - (*it).first;
				if (mn - v > eps) mn = v, res = (*it).second;
				else if (fabs(mn - v) < eps) res = min(res, (*it).second);
			}
		}
		printf("Case #%d: %d\n", ++ cnt, res - 1);
	}
	return 0;
}