Exploit stdin to call local function in C

Note: this is extra credit for a homework assignment

I'm trying to figure out how to call win() via stdin. I assumed that the vulnerability would be exploited by a buffer overflow, but I can't even figure out how to overflow the buffer (if I input a 45-character string, it prints all 45 characters). What sort of vulnerability should I be looking for here, and how would I go about exploiting it?

int win()
{
    printf("Great Job\n");
    exit(0);
}

int main()
{
    char buffer[40];
    unsigned long long target;

    scanf("%s", buffer);
    printf("You sent: %s\n", buffer);
    exit(1);
}