算概率：

#include <bits/stdc++.h>
using namespace std;

double cal(long long L, long long R) {
	long long fm = R - L + 1, fz = 0;
	for (long long l = 1, r = 1, i = 1; i <= 18; i++, l *= 10, r = r * 10 + 9) {
		if (r < L)
			continue;
		if (l > R)
			break;
		fz += min(R, r) - max(L, l) + 1;
	}
	return 1.0 * fz / fm;
}

int main() {
	cout << cal(3, 17) << endl;
}

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