def prime_check(n: str):
def is_prime_div(n: int):
if n <= 1:
return False
for i in range(2, int(n ** 0.5) + 1):
if n % i == 0:
return False
return True
out = []
if len(n) != 4:
return 0
for i in range(4):
for j in range(i + 1, 5):
if is_prime_div(int(n[i:j])):
out.append(int(n[i:j]))
return max(out) if len(out) > 0 else 0
print(prime_check(input()))