#include <bits/stdc++.h>
using namespace std;
vector<int> pos[256];
int main() {
int n, k;
string s;
cin >> n >> k;
cin >> s;
for ( int i = 0; i < n; i++ )
pos[s[i]].push_back( i );
int minChanges = n + 1;
int o = 0, i = 0;
for ( int j = 0; j < pos['J'].size() - k + 1; j++ ) {
while ( o < pos['O'].size() && pos['O'][o] < pos['J'][j + k - 1] )
o++;
if ( o + k - 1 >= pos['O'].size() )
break;
while ( i < pos['I'].size() && pos['I'][i] < pos['O'][o + k - 1] )
i++;
if ( i + k - 1 >= pos['I'].size() )
break;
int changes = pos['I'][i + k - 1] - pos['J'][j] + 1 - 3 * k;
minChanges = min( minChanges, changes );
}
cout << (minChanges == n + 1 ? -1 : minChanges);
return 0;
}
