Find Substring

#include <iostream>
#include <string.h>
using namespace std;

int find(char first_string[], char second_string[])
{
    for (int i = 0; i < strlen(first_string); i++)
    {
        int j;

        for (j = 0; j < strlen(second_string); j++)
        {
            if (first_string[i + j] != second_string[j])
            {
                break;
            }
        }

        if (j == strlen(second_string))
        {
            return i;
        }
    }

    return -1;
}

int main()
{
    char key[20] = "YoonA";
    char code[50] = "I love YoonA a lot";

    int where_is_it;
    where_is_it = find(code, key);

    cout << where_is_it;
}