#include <bits/stdc++.h>
#define int long long
using namespace std;
// costJ(i), costI(i)
// sol = min_{i, j} (j-i-1-K) + costJ[i] + costI[j]
signed main(void)
{
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int N, K;
string s;
cin >> N >> K >> s;
vector<int> costJ(N, 1e18), costI(N, 1e18);
vector<int> nbOcc(N);
for (int i(0); i < N; ++i)
{
if (i) nbOcc[i] += nbOcc[i-1];
nbOcc[i] += s[i] == 'O';
}
queue<int> q;
for (int i(0); i < N; ++i)
{
if (s[i] == 'J')
q.push(i);
while ((int)q.size() > K) q.pop();
if ((int)q.size() == K)
{
int lst = q.front();
costJ[i] = i - lst +1 - K;
}
}
for (int deb(0); deb < N; ++deb)
if (costJ[deb] < 1e18)
costJ[deb] = costJ[deb] - deb - 1 - K;
while (!q.empty()) q.pop();
for (int i(N-1); i >= 0; --i)
{
if (s[i] == 'I')
q.push(i);
while ((int)q.size() > K) q.pop();
if ((int)q.size() == K)
{
int lst = q.front();
costI[i] = lst - i + 1 - K;
}
}
for (int fin(N-1); fin >= 0; --fin)
if (costJ[fin] < 1e18)
{
costI[fin] += fin;
if (fin < N-1)
costI[fin] = min(costI[fin], costI[fin+1]);
}
int sol(1e18);
for (int deb(0); deb < N-1; ++deb)
{
if (nbOcc[N-2] - nbOcc[deb] < K) continue;
int lo(deb+1), up(N-1);
while (lo < up)
{
int mid = (lo + up) / 2;
if (nbOcc[mid-1] - nbOcc[deb] < K)
lo = mid+1;
else
up = mid;
}
int fin = lo;
sol = min(sol, costJ[deb] + costI[fin]);
}
if (sol == 1e18)
sol = -1;
cout << sol << endl;
}
