View Compilation Error

0_0_2083694_5547.cpp: In function `int main()':
0_0_2083694_5547.cpp:8: error: `bignum' undeclared (first use this function)
0_0_2083694_5547.cpp:8: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_2083694_5547.cpp:8: error: expected `;' before "sum"
0_0_2083694_5547.cpp:36: error: `sum' undeclared (first use this function)