string concatination

    #include <stdio.h>
    int main()
    {
        char s1[100], s2[100], i, j;
        printf("Enter first string: ");
        gets(s1);
        printf("Enter second string: ");
        gets(s2);

        for(i = 0; s1[i] != '\0'; ++i);
        for(j = 0; s2[j] != '\0'; ++j, ++i)
        {
            s1[i] = s2[j];
        }
        s1[i] = '\0';
        puts(s1);
        return 0;
    }