8503589813642624になったけど
#include <stdio.h>
#include <math.h>

#define SIZE 10
#define VAC 100000
typedef int bignum[SIZE];

void regular_bignum(bignum n)
{
int i;
for (i=0; i < SIZE-1; i++)
if (n[i] >= VAC) {
n[i+1] += n[i] / VAC;
n[i] %= VAC;
}
}

void write_bignum(bignum n)
{
int i = SIZE-1;

regular_bignum(n);

for (; i>=0; i--)
if (n[i] != 0) {
printf("%u", n[i]);
i--;
break;
}
for (; i>=0; i--)
printf("%05u", n[i] % VAC);
printf("\n");
}

void make_bignum(bignum b, int n)
{
int i;

b[0] = n;
for (i=1; i < SIZE; i++)
b[i] = 0;
}