#include <bits/stdc++.h>

using namespace std;

using ll = long long;

constexpr char newl = '\n';

// a * x + b * y == gcd(a, b) なるx, yを計算
// 返り値はgcd(a, b)
template<typename T>
T extgcd(T a, T b, T& x, T& y) {
    T d = a;
    if (b != 0) {
        d = extgcd(b, a % b, y, x);
        y -= (a / b) * x;
    } else {
        x = 1; y = 0;
    }
    return d;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int a, b;
    cin >> a >> b;
    
    if (__gcd(a, b) != 1) {
        cout << -1 << newl;
        return 0;
    }

    vector<int> dp(100001, false);
    dp[0] = true;

    for (int i = 0; i <= 100000; i++) {
        if (!dp[i]) continue;
        if (i + a <= 100000) dp[i + a] = true;
        if (i + b <= 100000) dp[i + b] = true;
    }
    
    int ans = 0;
    for (int i = 1; i <= 100000; i++) {
        ans += !dp[i];
    }
    cout << ans << newl;

    return 0;
}