foo.c: In function 'main':
foo.c:5:28: error: unknown type name 'bool'
  int n, num[100], ans = 0; bool sum[20000] = {0};
                            ^
foo.c:14:27: error: 'true' undeclared (first use in this function)
    sum[num[i] + num[j]] = true;
                           ^
foo.c:14:27: note: each undeclared identifier is reported only once for each function it appears in