#include <string.h>
#include <stdlib.h>
#include <stdio.h>

int main()
{
#define len 300
    char st1[len], st2[len];
    printf("1st string: ");
    fgets(st1, len, stdin);
    printf("2nd string: ");
    fgets(st2, len, stdin);
    if (strlen(st1) != strlen(st2))
    {
        fprintf(stderr,"strlen(st1) != strlen(st2)\n");
        return 1;
    }
    
    printf("\n\n1st string: %s",st1);
    printf("2nd string: %s",st2);
    
    for(int i = 1; i < strlen(st2); i+=2)
        st1[i] = st2[i];
    printf("Res string: %s",st1);
}