[codeforces 317]A. Perfect Pair

试题描述

Let us call a pair of integer numbers m-perfect, if at least one number in the pair is greater than or equal to m. Thus, the pairs (3, 3) and (0, 2) are 2-perfect while the pair (-1, 1) is not.

Two integers xy are written on the blackboard. It is allowed to erase one of them and replace it with the sum of the numbers, (x + y).

What is the minimum number of such operations one has to perform in order to make the given pair of integers m-perfect?

输入

Single line of the input contains three integers xy and m ( - 1018 ≤ xym ≤ 1018).

Please, do not use the %lld specifier to read or write 64-bit integers in C++. It is preffered to use the cin, cout streams or the %I64dspecifier.

输出

Print the minimum number of operations or "-1" (without quotes), if it is impossible to transform the given pair to the m-perfect one.

输入示例

-  

输出示例


数据规模及约定

见“输入

题解

如果都是整数,那么不难发现增长率和斐波那契数列相同,就是指数级的,所以直接模拟即可。注意特判有负数的情况。如果只有一个负数,先把这个负数加成正的再模拟。如果两个都是负数,那么不可能再变大了。

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <stack>
#include <vector>
#include <queue>
#include <cstring>
#include <string>
#include <map>
#include <set>
using namespace std;
#define LL long long const int BufferSize = 1 << 16;
char buffer[BufferSize], *Head, *Tail;
inline char Getchar() {
if(Head == Tail) {
int l = fread(buffer, 1, BufferSize, stdin);
Tail = (Head = buffer) + l;
}
return *Head++;
}
LL read() {
LL x = 0, f = 1; char c = getchar();
while(!isdigit(c)){ if(c == '-') f = -1; c = getchar(); }
while(isdigit(c)){ x = x * 10 + c - '0'; c = getchar(); }
return x * f;
} LL x, y, m; int main() {
x = read(); y = read(); m = read(); if(x > y) swap(x, y);
if(x <= 0 && y <= 0 && y < m) return puts("-1"), 0;
if(y >= m) return puts("0"), 0;
LL cnt = abs(x) % y ? abs(x) / y + 1 : abs(x) / y;
x += cnt * y; if(x > y) swap(x, y);
while(y < m) {
x = x + y;
if(x > y) swap(x, y);
cnt++;
} printf("%I64d\n", cnt); return 0;
}

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