var1 = input() n, k = int(var1.split(" ")[0]), int(var1.split(" ")[1]) s = input() def check(n, k, s): ind = 0 j_count, o_count, i_count = 0, 0, 0 while (j_count < k) and (ind < len(s)): if s[ind] == "J": j_count += 1 ind += 1 while (o_count < k) and (ind < len(s)): if s[ind] == "O": o_count += 1 ind += 1 while (i_count < k) and (ind < len(s)): if s[ind] == "I": i_count += 1 ind += 1 if (j_count != k) or (o_count != k) or (i_count != k): return -1 else: return 1 if check(n, k, s[1:-1]) == 1: print(len(s)-2-k*3) elif check(n, k, s[1:]) == 1: print(0) elif check(n, k, s) == 1: print(0) else: print(-1)