int n;

char str[500005];

scanf("%d %s", &n, str);

int len = (int)(log((double)n) / log(2.0) + 1);

int size = (1 << (len + 1)) - 1;

vector<bool> mm(size, false);

for (int d = 1; d <= len; ++d) {

for (int i = 0; i < n + 1 - d; ++i) {

int now = 0;

for (int j = 0; j < d; ++j) {

if (str[i + j] == 'a') now = now * 2 + 1;

else now = now * 2 + 2;

}

mm[now] = true;

}

}

for (int i = 1; ; ++i) {

if (!mm[i]) {

int ret = 0;

deque<char> out;

while (i) {

if (i % 2) out.push_back('a');

else out.push_back('b');

i = (i - 1) / 2;

++ret;

}

printf("%d\n", ret);

for (int j = out.size() - 1; j >= 0; --j) printf("%c", out[j]);

printf("\n");

return 0;

}

}

return 0;