#include <bits/stdc++.h>
#define int long long
using namespace std;
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(nullptr);
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector<int> prefJ(n, 0), prefO(n, 0), prefI(n, 0);
vector<int> nxtJ(n, -1), nxtO(n, -1), nxtI(n, -1);
int lastJ = -1, lastO = -1, lastI = -1;
for (int i = 0; i < n; i++){
if (s[i] == 'J'){
prefJ[i]++;
if (lastJ != -1){
nxtJ[lastJ] = i;
}
lastJ = i;
}
if (s[i] == 'O'){
prefO[i]++;
if (lastO != -1){
nxtO[lastO] = i;
}
lastO = i;
}
if (s[i] == 'I'){
prefI[i]++;
if (lastI != -1){
nxtI[lastI] = i;
}
lastI = i;
}
}
for (int i = 1; i < n; i++){
prefJ[i] += prefJ[i-1];
prefO[i] += prefO[i-1];
prefI[i] += prefI[i-1];
}
// fix l, for once to get is
// if push l up 1, then furthest j drops and jumps to the closest one, same with o
// solve for l = 0 first
int firstJ = -1, firstO = -1, firstI = -1;
int farJ = 0, farO = 0, farI = 0;
int curJ = 0, curO = 0, curI = 0;
for (int i = 0; i < n; i++){
if (s[i] == 'J' && curJ < k){
if (firstJ == -1){
firstJ = i;
}
curJ++;
farJ = i;
}
if (s[i] == 'O' && curO < k && curJ == k){
if (firstO == -1){
firstO = i;
}
curO++;
farO = i;
}
if (s[i] == 'I' && curI < k && curO == k){
if (firstI == -1){
firstI = i;
}
curI++;
farI = i;
}
}
if (curI < k){
cout << -1;
return 0;
}
int ans = farI - firstJ + 1 - 3*k;
while(nxtJ[farJ] != -1){
int newFarJ = nxtJ[farJ];
int newFirstJ = nxtJ[firstJ];
int numO = prefO[newFarJ] - (farJ == 0 ? 0 : prefO[farJ-1]);
int newFarO = farO;
int newFirstO = firstO;
while(numO > 0 && nxtO[newFarO] != -1){
newFarO = nxtO[newFarO];
newFirstO = nxtO[newFirstO];
numO--;
}
if (numO > 0) break;
int numI = prefI[newFarO] - (farO == 0 ? 0 : prefO[farO-1]);
int newFarI = farI;
int newFirstI = firstI;
while(numI > 0 && nxtI[newFarI] != -1){
newFarI = nxtI[newFarI];
newFirstI = nxtI[newFirstI];
}
if (numI > 0) break;
firstJ = newFirstJ, firstO = newFirstO, firstI = newFirstI;
farJ = newFarJ, farO = newFarO, farI = newFarI;
ans = min(ans, farI - firstJ + 1 - 3*k);
}
cout << ans;
}
