#include<stdio.h>

int main()
{
    long input;
    int output[100];
    int i,j=0;
    scanf("%ld",&input);
    if (input<10)
    {
        printf("1%d",input);
        return 1;
    }
    while (input>9)
    {
        for(i=9;i>1;i--)
        {
            if(input%i == 0)
            {
                output[j]=i;
                printf("%d\n",i);
                j++;
                input/=i;
                break;
            }
        }
        if(i==1)
        {
            printf("-1");
            return 1;
        }
    }
    output[j]=input;
    for(i=j;i>=0;i--)
    {
        printf("%d",output[i]);
    }
    return 1;
}

Test:
51
Right answer:
-1

Test:
6
Right answer:
6

your output format is wrong
commentary //printf("%d\n",i);