The U.S. Department of State has divided all countries in the world into 4 categories by security level for recommendations for visiting by their citizens. The highest security level is 1, which includes the United States, Canada and a number of others. The lowest security level is 4 (countries that are dangerous and not recommended to visit), which, for example, include Ukraine and the Russian Federation for obvious reasons.
It is nice to know that Bulgaria has the first security group, which is higher than many European countries (Germany, France, Italy, England, Spain, etc.) that have security group 2.