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#include <stdio.h>

int main()
{
    int n = 0;
    scanf("%d", &n);

    int max_digit = -1;

    for (; n; n /= 10)
    {
        const int digit = n % 10;

        if (digit % 3 == 0 && digit > max_digit)
            max_digit = digit;
    }

    max_digit >= 0 ? printf("%d", max_digit) : puts("NO");
    return 0;
}