这里提供一个不用龟速乘的方法，用 __int128 写。

#include<bits/stdc++.h>
#define int long long
#define I_love_ch ios::sync_with_stdio(0)
#define China cin.tie(0)
#define France cout.tie(0)
#define ch_France return 0
using namespace std;
void read(__int128& x){
	int f=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){
		if(ch=='-') f=-1;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9'){
		x=(x<<1)+(x<<3)+(ch^48);
		ch=getchar();
	}
	x*=f;
}
void print(__int128 x){
	if(x==0){
		cout<<"0\n";
		return;
	}
	bool f=0;
	if(x<0){
		f=1;
		x=-x;
	}
	string s;
	while(x){
		s.push_back(x%10+'0');
		x/=10;
	}
	if(f) cout<<"-";
	reverse(s.begin(),s.end());
	cout<<s<<"\n";
}
__int128 a,b,p;
signed main(){
	I_love_ch;
	China;
	France;
	read(a);
	read(b);
	read(p);
	int ans=((a%p)*(b%p))%p;
	print(ans);
	ch_France;
}