l=Scanf_Int();
r=Scanf_Int();
m=Scanf_Int();
m1=Scanf_Int();
m2=0;
int z=1,z1=0,m4=0;
while (z!=z1){
ans++;
if(ans==100000000){
z=-1;
break;
}
z=r+m2*l;
z1=m1+m4*m;
if (z>z1){
m4++;
}
else {
m2++;
}
}
out.println(z);