#define _ISOC99_SOURCE
#include <stdlib.h>
long long atoll(const char *nptr);
The atoll() function converts the initial portion of the string pointed to by nptr to a 'long long int'. This is equivalent to strtoll(nptr, (char **)NULL, 10).
If successful, atoll() returns the converted signed long long value, represented in the string. If unsuccessful, it returns an undefined value.