Count Substring

def Count_Substring(STR, SUB_STR):
    SUB_LEN = len(SUB_STR)
    Counter = 0
    for i in range (len(STR)):
        if STR[i] == SUB_STR[0]:
            TEMP = STR[i:i+SUB_LEN]
            if TEMP == SUB_STR:
                Counter += 1

    return Counter