def main():
    import sys
    n = int(sys.stdin.readline())
    s = sys.stdin.readline().strip()

    # Step 1: Count 3,5,7 and collect remaining characters
    count = 0
    remaining = []
    for c in s:
        if c in {'3', '5', '7'}:
            count += 1
        else:
            remaining.append(c)
    
    n_remain = len(remaining)
    used = [False] * n_remain

    # Step 2: Greedily find '19' in remaining
    i = 0
    while i < n_remain - 1:
        if not used[i] and not used[i+1] and remaining[i] == '1' and remaining[i+1] == '9':
            count += 1
            used[i] = used[i+1] = True
            i += 2
        else:
            i += 1

    # Step 3: Greedily find 3-digit primes in remaining
    three_digit_primes = {'191', '199', '911', '919', '991'}
    i = 0
    while i <= n_remain - 3:
        if not used[i] and not used[i+1] and not used[i+2]:
            substr = ''.join(remaining[i:i+3])
            if substr in three_digit_primes:
                count += 1
                used[i] = used[i+1] = used[i+2] = True
                i += 3
            else:
                i += 1
        else:
            i += 1

    print(count)

if __name__ == '__main__':
    main()