View Compilation Error

0_0_17018113_24816.cpp: In function 'int main()':
0_0_17018113_24816.cpp:24:12: error: invalid conversion from 'C*' to 'size_t {aka long long unsigned int}' [-fpermissive]
   qsort(a,a+m*n,sizeof(a[0]),cmp);
            ^
In file included from GCC4.9.2/x86_64-w64-mingw32/include/c++/cstdlib:72:0,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/bits/stl_algo.h:59,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/algorithm:62,
                 from 0_0_17018113_24816.cpp:4:
GCC4.9.2/x86_64-w64-mingw32/include/stdlib.h:353:16: note: initializing argument 2 of 'void qsort(void*, size_t, size_t, int (*)(const void*, const void*))'
   void __cdecl qsort(void *_Base,size_t _NumOfElements,size_t _SizeOfElements,int (__cdecl *_PtFuncCompare)(const void *,const void *));
                ^