Submission

Score: 100

User: loop

Problemset: อนุกรม

Language: cpp

Time: 0.004 second

Submitted On: 2026-10-10 18:29:43

#include <iostream>
#include <string>
using namespace std;

// F(100) has 21 digits, more than unsigned long long holds, so add decimal strings.
string add(const string &a, const string &b) {
    string r = "";
    int i = (int)a.size() - 1, j = (int)b.size() - 1, carry = 0;
    while (i >= 0 || j >= 0 || carry) {
        int d = carry;
        if (i >= 0) d += a[i--] - '0';
        if (j >= 0) d += b[j--] - '0';
        r = char('0' + d % 10) + r;
        carry = d / 10;
    }
    return r;
}

int main() {
    int n;
    cin >> n;
    string a = "1", b = "1";
    for (int k = 3; k <= n; k++) {
        string c = add(a, b);
        a = b;
        b = c;
    }
    cout << (n <= 1 ? a : b) << '\n';
    return 0;
}