O-lucky numbers
Everybody knows that lucky numbers are positive integers whose decimal record contains only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.
Find sum of digits for all N-digit numbers.
InputFrom first and only line of input read integer N, (N <= 10000) number of digits.
OutputIn first and only line of output, write solution. Sum Of digits for all n-digit numbers.
NOTICE:
20% : N<10
40% : N<20
60% : N<100
80% : N<1000
Input:
Output:
1
Output:
11
Input:
Output:
3
Output:
132
EXPLANATION: First example 4+7 =11
Second example: 3-digit lucky numbers are: 444,447,474,477,744,747,774,777.
Solution is: 12+15+15+18+15+18+18+21=132
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