460.cpp

#include <cstdio>

int count5(long long n) {
    int answer = 0;
    while (n > 0) {
        answer += (n % 10 == 5);
        n /= 10;
    }
    return answer;
}

long long count5_before(long long n) {
    if (n < 5) {
        return 0;
    } else if (n <= 9) {
        return 1;
    } else if (n % 10 == 9) {
        return (n+1) / 10 + 10 * count5_before((n-9) / 10);
    } else {
        return count5(n) + count5_before(n-1);
    }
}

int main() {
    long long n;
    scanf("%lld", &n);
    printf("%lld", count5_before(n));
    return 0;
}