#include <bits/stdc++.h>
using namespace std;
string s;
vector<int> nxt1, nxt2, nxt3;
int sum = 0, wyn = 0;
int p1 = 0, p2 = 0, p3 = 0;
int k1 = 0, k2 = 0, k3 = 0;
bool pos = true;
void push3(){
p3 = nxt3[p3];
k3 = nxt3[k3];
if(p3==-1){
pos = false;
}
}
void push2(){
p2 = nxt2[p2];
k2 = nxt2[k2];
if(p2==-1){
pos = false;
}
}
void push1(){
p1 = nxt1[p1];
k1 = nxt1[k1];
if(p1==-1){
pos = false;
}
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(nullptr); cout.tie(nullptr);
int n, k; cin>>n>>k;
nxt1.resize(n, -1);
nxt2.resize(n, -1);
nxt3.resize(n, -1);
cin>>s;
for(int i = n-1; i > 0; i--){
nxt1[i-1] = nxt1[i];
nxt2[i-1] = nxt2[i];
nxt3[i-1] = nxt3[i];
switch(s[i]){
case 'J':
nxt1[i-1] = i;
break;
case 'O':
nxt2[i-1] = i;
break;
case 'I':
nxt3[i-1] = i;
break;
}
}
p1 = nxt1[0];
p2 = nxt2[0];
p3 = nxt3[0];
k1 = nxt1[0];
k2 = nxt2[0];
k3 = nxt3[0];
switch(s[0]){
case 'J':
p1 = 0;
k1= 0;
break;
case 'O':
p2 = 0;
k2= 0;
break;
case 'I':
p3 = 0;
k3= 0;
break;
}
for(int i = 0; i < k-1; i++){
p1 = nxt1[p1];
p2 = nxt2[p2];
p3 = nxt3[p3];
if(p1==-1) {cout<<-1; return 0;}
if(p2==-1) {cout<<-1; return 0;}
if(p3==-1) {cout<<-1; return 0;}
}
while(p1>k2){
push2();
if(!pos) {cout<<-1; return 0;}
}
while(p2>k3){
push3();
if(!pos) {cout<<-1; return 0;}
}
int wyn = p3-k1;
while(pos){
push1();
while(p1>k2){
push2();
if(!pos) {break;}
}
while(p2>k3){
push3();
if(!pos) {break;}
}
if(pos) wyn = min(wyn, p3-k1);
}
cout<<wyn+1-3*k;
return 0;
}
