#include <cstdint>
#include <iostream>

int main() {
    int32_t r, c;
    std::cin >> r >> c;
    int32_t ans = -1;
    if (c == 1) {
        ans = 1;
    } else if (r == 1) {
        ans = (c + 2) / 3;
    } else if (r == 2) {
        ans = c / 2 + 1;
    }

    std::cout << ans << '\n';
}