#include <bits/stdc++.h>
using namespace std;
int n, k;
int cmin = INT_MAX;
string str;
int firstpos[3][300005];
int qs[3][300005];
int ord(char x) {
if(x == 'J') return 0;
if(x == 'O') return 1;
return 2;
}
int main() {
memset(firstpos, -1, sizeof firstpos);
cin.tie(0) -> sync_with_stdio(0);
cin >> n >> k;
cin >> str;
for(int i=1;i<=n;i++) {
for(int k=0;k<3;k++) qs[k][i] = qs[k][i-1];
qs[ord(str[i-1])][i]++;
}
for(int i=n;i>=1;i--) {
for(int k=0;k<3;k++) {
firstpos[k][qs[k][i]] = i;
}
}
for(int i=0;i<=n;i++) {
int cpos = i;
for(int j=0;j<3;j++) {
cpos = (firstpos[j][qs[j][cpos] + k]);
if(cpos == -1) break;
}
if(cpos != -1) cmin = min(cmin, cpos - i);
}
cout << (cmin > n ? -1 : cmin - k*3);
}
