6

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

void concat(char * a, char * b, int ind){
    if (ind > strlen(a)){
        return NULL;
    }
    char c[100] = "";
    int i = 0;
    for (i = 0; i < ind; i++){
        c[i] = a[i];
    }
    for (int j  =0; j < strlen(b); j++){
        c[i] = b[j];
        i++;
    }
    for (int j = ind; j < strlen(a); j++){
        c[i] = a[j];
        i++;
    }
    c[i] = '\0';
    strcpy(a, c);
}

int main()
{
    char a[100] = "11111";
    char b[100] = "222";
    concat(a, b, 3);
    printf("%s", a);

    return 0;
}