#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e5+5;
int pref[3][MAXN],dp[3][MAXN];
int main(){
int N,K; cin >> N >> K;
for(int i = 1;i<=N;++i){
char c; cin >> c;
if(c == 'J') pref[0][i]++;
else if(c == 'O') pref[1][i]++;
else pref[2][i]++;
pref[0][i] += pref[0][i-1];
pref[1][i] += pref[1][i-1];
pref[2][i] += pref[2][i-1];
}
memset(dp[0],0x3f,sizeof(dp[0]));
int cur = 1e9, pos = 0;
for(int i = 1;i<=N;++i){
while(pref[0][i]-pref[0][pos] >= K && pos < i){
cur = min(cur,-pos);
pos++;
}
dp[0][i] = cur+i;
}
memset(dp[1],0x3f,sizeof(dp[1]));
cur = 1e9; pos = 0;
for(int i = 1;i<=N;++i){
while(pref[1][i]-pref[1][pos] >= K && pos < i){
cur = min(cur,dp[0][pos]-pos);
pos++;
}
dp[1][i] = cur+i;
}
memset(dp[2],0x3f,sizeof(dp[2]));
cur = 1e9; pos = 0;
for(int i = 1;i<=N;++i){
while(pref[2][i]-pref[2][pos] >= K && pos < i){
cur = min(cur,dp[1][pos]-pos);
pos++;
}
dp[2][i] = cur+i;
}
int ans = *min_element(dp[2],dp[2]+N+1) - 3*K;
if(ans > 1e7) cout << -1 << "\n";
else cout << ans << "\n";
return 0;
}
