unsigned long long latoi(char * s) {
   unsigned long long num = 0;
   while(*s>='0' && *s<='9') num = num * 10 + *(s++) - '0';
   return num;
}

int main() {
   char* s = "798654564654564654";
   printf("%lld",latoi(s));
   return 0;
}

798654564654564654