Code:

#include <stdio.h>
#include <string.h>
const int SIZE = 2005;
char a[SIZE], b[SIZE];
unsigned long long c[2*SIZE];

int main()
{
	int sizea, sizeb, started = 0;
	scanf("%s %s", a, b);
	sizea = strlen(a);
	sizeb = strlen(b);
	for (int i = 0; i < sizea; i++)
	{
		a[SIZE-sizea+i] = a[i] - '0';
		a[i] = 0;
    }
	for (int i = 0; i < sizeb; i++)
	{
		b[SIZE-sizeb+i] = b[i] - '0';
		b[i] = 0;
	}
	for (int i = 0; i < sizea; i++)
	    for (int j = 0; j < sizeb; j++)
	    {
	        c[2*SIZE-i-j-1] += a[SIZE-i-1] * b[SIZE-j-1];
	        c[2*SIZE-i-j-2] += c[2*SIZE-i-j-1] / 10;
	        c[2*SIZE-i-j-1] %= 10;
	    }
	for (int i = 0; i < 2*SIZE; i++)
	{
		started |= c[i] != 0;
		if (started)
		    putchar(c[i] + '0');
	}
	if (!started) putchar('0');
	return 0;
}