http://projecteuler.net/problem=58

**Spiral primes**

Starting with 1 and spiralling anticlockwise in the following way, a square spiral with side length 7 is formed.

~~36 35 34 33 32~~3731

38~~16 15 14~~17~~30~~13

39 18~~4~~5~~12 29~~3

40 19 6 1 2 11 28

41 20~~8 9 10 27~~7

42 21 22 23 24 25 26

~~44 45 46 47 48 49~~43It is interesting to note that the odd squares lie along the bottom right diagonal, but what is more interesting is that 8 out of the 13 numbers lying along both diagonals are prime; that is, a ratio of 8/13 62%.

If one complete new layer is wrapped around the spiral above, a square spiral with side length 9 will be formed. If this process is continued, what is the side length of the square spiral for which the ratio of primes along both diagonals first falls below 10%?