#include <stdio.h>

int gcd(int a, int b) {
	int r;
	while (b != 0) r = a % b, a = b, b = r;
	return a;
}

int lcm(int a, int b) {
	return a/gcd(a, b)*b;
}

char f[10004];
int main()
{
    int A, B, x, y, N, n;
    int ans; 

    scanf("%d%d", &A, &B);
    if (gcd(A, B) > 1) puts("-1");
    else {
        N = lcm(A, B);
        for (x = 0; x <= N; ++x) for (y = 0; y <= N; ++y) {
            n = A*x + B*y;
            if (n < N) f[n] = 1;
        }
        ans = 0;
        for (x = 1; x < N; ++x) if (!f[x]) ++ans;
        printf("%d\n", ans);
    }
    return 0;
}