#include <iostream>
#include <vector>
#include <cmath>

const int INF = 1e8;
using std::vector;
using std::min;
using std::max;

int main() {
    int level, phones;
    std::cin >> level >> phones;
    vector<vector<int>> dp(level + 1, std::vector<int>(phones + 1));
    for (int lev = 1; lev <= level; ++lev) {
        dp[lev][1] = lev - 1;
        dp[lev][0] = INF;
    }

    for (int lev = 2; lev <= level; ++lev) {
        for (int phones_cnt = 2; phones_cnt <= phones; ++phones_cnt) {
            int best = INF;
            int left = 1;
            int right = lev - 1;
            while (left + 1 != right and left != right) {
                int middle = (left + right) >> 1;
                if (dp[lev - middle][phones_cnt] < dp[middle][phones_cnt-1]) {
                    right = middle;
                }
                else {
                    left = middle;
                }
            }
            best = min(max(dp[lev - right][phones_cnt], dp[right][phones_cnt-1]), max(dp[lev - left][phones_cnt], dp[left][phones_cnt-1]));
            dp[lev][phones_cnt] = 1 + best;
        }
    }

    if (dp[level][phones] == INF) {
        std::cout << -1;
    } else {
        if (phones >= log2(level)) {
            std::cout << ceil(log2(level));
        } else {
            std::cout << dp[level][phones];
        }
    }
    return 0;
}