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#include <stdio.h>

int convert(char* number, int sum) {
	if (*number == '\0') return sum; //sind am Ende angekommen
	int digit = *number - '0'; //aktuelle Ziffer
	return convert(number + 1, 2*sum+digit);
}

int main(void) {
	char number[] = "100101";
	printf("%d\n", convert(number, 0));
}