# numbers are represented in unary using \e's
# \a is a separator
# start with 100000000 
$_=("\e"x100000000)."\a";
# write down all the numbers from 2 to 100000000 
while(s/^\e(\e{2,})/\1\a\e\1/) {}
# use the Sieve of Eratosthenes to eliminate the composite numbers
while(s/(?<!\e)(\e+)(?!\e)(.*)\a\1+(?!\e)/\1\2/g) {}
s/\e//g;
# print the number of primes less than 100000000,
# which is 5761455
print (length $_);
print "\n";