38 lines
847 B
Rust
38 lines
847 B
Rust
use std::io::{read_to_string, stdin};
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const MOD: usize = 1_000_000_007;
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fn get_mod_power(n: usize, p: usize) -> usize {
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let mut x = p;
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let mut curr = n % MOD;
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let mut res = 1;
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while x > 0 {
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if x & 1 == 1 {
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res *= curr;
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res %= MOD;
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}
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x >>= 1;
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curr *= curr;
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curr %= MOD;
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}
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res
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}
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fn main() {
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let temp = read_to_string(stdin()).unwrap();
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let mut iter = temp
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.split_ascii_whitespace()
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.map(|x| x.parse::<usize>().unwrap());
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let m = iter.next().unwrap();
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let dices = (0..m)
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.map(|_| (iter.next().unwrap(), iter.next().unwrap()))
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.collect::<Vec<_>>();
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let res = dices
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.iter()
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.map(|&(n, s)| s * get_mod_power(n, MOD - 2) % MOD)
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.sum::<usize>()
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% MOD;
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println!("{}", res);
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}
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