Incomplete

#include<stdio.h>
#include<string.h>
int main()
{
    char s1[50],s2[50],s3[50];
    int i,j,k,l,m;
    printf("Enter first string: ");
    gets(s1);
    printf("Enter second string: ");
    gets(s2);
    char S1[50],S2[50];
    strcpy(s1,S2);
    strcpy(s2,S2);
    k=0;
    for(i=0,j=0;S1[i]!='\0';)
    {
        if(S1[i]==S2[j])
        {
            s3[k]=S1[i];
            k++;
            for(l=i;S1[i]!='\0';l++)
            {
                S1[i]=S1[i+1];
            }
            for(m=j;S2[m]!='\0';m++)
            {
                S2[m]=S2[m+1];
            }

        }
        if(s1[i]!=s2[j])
        {
            for(l=j;s2[l]!='\0';l++)
            {
                if(S1[i]==S2[l])
                {
                    s3[k]=S1[i];
                    k++;


                    break;
                }
            }
        }
    }
    s3[k]='\0';
    printf("%s", s3);
}