View Compilation Error

0_0_9750287_8543.cpp: In function `long long int mul(long long int, long long int)':
0_0_9750287_8543.cpp:23: error: `__int128' undeclared (first use this function)
0_0_9750287_8543.cpp:23: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_9750287_8543.cpp:23: error: expected `;' before "a"