string _multi(string a,string b){
	int lena = a.size(),lenb = b.size(),lenc = lena + lenb;
	int a1[lenc + 1],b1[lenc + 1],c1[lenc + 1];
	memset(a1,0,sizeof(a1));
	memset(b1,0,sizeof(b1));
	memset(c1,0,sizeof(c1));
	for (int i = 1; i <= lena; i++) a1[i] = a[lena - i] - '0';
	for (int i = 1; i <= lenb; i++) b1[i] = b[lenb - i] - '0';
	for (int i = 1; i <= lena; i++) 
		for (int j = 1; j <= lenb; j++) 
			c1[i + j - 1] += a1[i] * b1[j];
	for (int i = 1; i <= lenc; i++) {
		if (c1[i] >= 10) {
			c1[i + 1] += c1[i] / 10;
			c1[i] %= 10;
		}
	}
	while (c1[lenc] == 0 && lenc > 1) {
		lenc--;
	}
	string c;
	for (int i = 0; i < lenc; i++) {
		c[i] = c1[lenc - i] + '0';
	}
	return c;
}