#include<bits/stdc++.h>
using namespace std;
 
// #define DEBUG
#define fi first
#define se second
#define be begin
#define en end
#define pb push_back 
#define mp make_pair 
#define ALL(a) (a).be() , (a).en()
#define REP(i,n) for(int (i)=0;(i)<(n);(i)++)        //repeat n times
#define REP2(i,s,n) for(int (i)=(s);(i)<(n);(i)++)   //repeat from s to n 
#define REPD(i,n) for(int (i)=(n);(i)>=0;(i)--)      //repeat from n to 0
#define REPD2(i,s,e) for(int (i)=(s);(i)>=(e);(i)--) //repeat from s to e
#define ASIZE(a) (sizeof(a) / sizeof(a[0]))          //array size
#define SORTD(a) sort(a,a+ASIZE(a),greater<int>())   //sort in descending order for array
#define SORTA(a) sort(a,a+ASIZE(a))                  //sort in ascending order for array
#define SORTVD(a) sort(ALL(a),greater<int>())        //sort in descending order for vector type
#define SORTVA(a) sort(ALL(a))                //sort in ascending order for vector type
 
using LL = long long;
 
template<typename T1,typename T2> using P = pair< T1, T2 >;
using Pii = P<int,int>;
using Pdd = P<double,double>;
 
template<typename T> using V = vector< T >;
using Vi = V<int>;
using Vll = V<LL>;
using Vs = V<string>;
 
template<typename T1,typename T2> using M = map< T1, T2>;
using Mii = M<int, int>;
using Msi = M<string, int>;
 
const int MOD = 1000000007;
const int INF = 1999999999;
const LL INFLL = 999999999999999;
 
const int DX4[4]= {-1,0,1,0};
const int DY4[4]= {0,-1,0,1};
 
const double PI=3.141592653589793;
 
//scan
void SCAN(int *a){scanf("%d",a);}                             //scan for type int
void SCAN(int *a,int n){int i;REP(i,n){scanf("%d",&a[i]);}}   //scan for type int array
void SCAN(Pii *a){scanf("%d",&a->fi);scanf("%d",&a->se);}     //scan for type pair<int, int>
void SCAN(LL *a){scanf("%lld",a);}                            //scan for type long long int
void SCAN(LL *a,int n){int i;REP(i,n){scanf("%lld",&a[i]);}}  //scan for type long long int array
void SCAN(char *c){scanf(" %c",c);}                           //scan for type char
void SCAN(char *c,int n){int i;REP(i,n){scanf(" %c",&c[i]);}} //scan for type char array
 
//print 
void PRINT(int a){printf("%d\n",a);}
void PRINT(int *a,int s,char c='\n'){int i;REP(i,s){if(i==s-1){c='\n';}printf("%d%c",a[i],c);}}
void PRINT(Vi a,char c='\n'){int i;REP(i,a.size()){if(i==a.size()-1){c='\n';}printf("%d%c",a[i],c);}}
void PRINT(LL a){printf("%lld\n",a);}
void PRINT(LL *a,int s,char c='\n'){int i;REP(i,s){if(i==s-1){c='\n';}printf("%lld%c",a[i],c);}}
void PRINT(double a){printf("%.15f\n",a);}
void PRINT(double *a,int s,char c='\n'){int i;REP(i,s){if(i==s-1){c='\n';}printf("%f%c",a[i],c);}}
void PRINT(char a){printf("%c\n",a);}
void PRINT(string a){printf("%s\n",a.c_str());}
 
//sort and remove duplicates for type vector
template<typename A>
void UNIQUE(vector< A > &a,int mode=0){
	if(mode==0){
		sort(ALL(a),greater< A >());
	}else{
		sort(ALL(a));
	}
	a.erase(unique(ALL(a)),a.en());
}
 
//fill an N-dimensional array with val
template<typename A, size_t N, typename T>
void FILL(A (&array)[N], const T &val){
	fill((T*)array,(T*)(array+N),val);
}
 
long double pascalTri(int n,int r){
	long double tri[n+1][n+1];
	int i,j;
	REP(i,n+1){REP(j,n+1){tri[i][j]=0;}}
	REP(i,n+1){
		REP(j,n+1){
			if(j>i){break;}
			if(j==0||j==i){tri[i][j]=1;}else{tri[i][j]=(tri[i-1][j-1]+tri[i-1][j]);}
		}
	}
	return tri[n][r];
}
 
LL GCD(LL a,LL b){
	LL t;
	if(a<b){t=a;a=b;b=t;}
	if(b==0){return a;}
	LL r;
	while(a%b!=0){r=a%b;a=b;b=r;}
	return b;
}
 
LL LCM(LL a,LL b){LL ab=(a*b)%MOD;return ab/GCD(a%b,b);}
 
double area(Pii a,Pii b,Pii c){
	return ((b.fi-a.fi)*(c.se-a.se)-(b.se-a.se)*(c.fi-a.fi))/2.;
}
 
int checkCross(Pii v1,Pii v2,Pii a,Pii b){
	// cout<<area(v1)<<" "<<area(v2)<<endl;
	if(area(a,b,v1)*area(a,b,v2)>=0){return 0;}
	if(area(v1,v2,a)*area(v1,v2,b)>=0){return 0;}
	return 1;
}
 
const int csize=1;
int dist[csize][csize];
void floyd(int n){
	int i,j,k;
	REP(k,n){REP(i,n){REP(j,n){dist[i][j]=min(dist[i][j],dist[i][k]+dist[k][j]);}}}
}
 
//Bipartite method : x^n
LL BMPow(int x,int n,int m=0){
	LL ans=1;LL p=x;
	if(m==0){
		while(n>0){if(n&1==1){ans*=p;} p*=p; n>>=1;}
	}
	else{
		while(n>0){if(n&1==1){ans=(ans*p)%m;} p=(p*p)%m ;n>>=1;}
	}
	return ans;
}
 
//x^-1=x^(m-2) mod m
LL modInv(LL x,int m){return BMPow(x,m-2,m);}
 
//x! or x! mod m
LL factorial(int x,int m=0){
	int i; LL a=1;
	if(m==0){REPD2(i,x,1){a*=i;}}else{REPD2(i,x,1){a=(a*i)%m;}}
	return a;
}
 
//centroid of polygon
Pdd centroidPolygon(Pii *a,int n){
	Pdd G; G.fi=0.; G.se=0.;
	REP(i,n){ G.fi+=a[i].fi; G.se+=a[i].se; }
	G.fi/=n;G.se/=n;
	return G;
}
 
// sign of val
template <typename T> 
int sgn(T val) {
	return (T(0) < val) - (val < T(0));
}
 
 
 
int main(){
	// cin.tie(0);
	// ios::sync_with_stdio(false);
	int i,j;
	LL n,a,b,c;

	SCAN(&a);SCAN(&b);
 
	
	#ifdef DEBUG
		auto start=chrono::system_clock::now();
	#endif

	if(a%b==0){
		PRINT(-1);
		return 0;
	}
	
	// if(a*(b-1))
	PRINT(a*(b-1));
	
 
	#ifdef DEBUG
		auto end=chrono::system_clock::now();
		cout<<"Elapsed time : "<<chrono::duration_cast<chrono::milliseconds>(end-start).count()<<"ms."<<endl;
	#endif
 
	return 0;
}

Measured memory_bytes: 256