Lucky Balance

Chef has the string s of length n consisted of digits 4 and 7. The string s is called balanced if there exits such integer x (1 ≤ x ≤ n) that the number of digits 4 in substring s[1; x) is equal to the number of digits 7 in substring s(x; n], where s[1; x) is the substring from the 1st digit to (x-1)th digit of s, and s(x; n] is the substring from the (x+1)th digit to nth digit of s. For example,