Problem:
Problem A: Hanafuda Shuffle

There are a number of ways to shuffle a deck of cards. Hanafuda
shuffling for Japanese card game 'Hanafuda' is one such example.
The following is how to perform Hanafuda shuffling.

There is a deck of n cards. Starting from the p-th card
from the top of the deck, c cards are pulled out
and put on the top of the deck, as shown in Figure 1.
This operation, called a cutting operation, is repeated.

Write a program that simulates Hanafuda shuffling and answers which
card will be finally placed on the top of the deck.

Figure 1: Cutting operation

Input

The input consists of multiple data sets.
Each data set starts with a line containing two positive integers n
(1 <= n <= 50) and r (1 <= r <= 50); n and
r are the number of cards in the deck and the number of cutting
operations, respectively.

There are r more lines in the data set, each of which
represents a cutting operation. These cutting operations
are performed in the listed order.
Each line contains two positive integers p and c
(p + c <= n + 1).
Starting from the p-th card from the top of the deck, c
cards should be pulled out and put on the top.

The end of the input is indicated by a line which contains two zeros.

Each input line contains exactly two integers separated by a space
character.
There are no other characters in the line.

Output

For each data set in the input, your program
should write the number of the top card after the shuffle.
Assume that at the beginning the cards
are numbered from 1 to n, from the bottom to the top.
Each number should be written in a separate line
without any superfluous characters such as leading or following spaces.

Sample Input

5 2
3 1
3 1
10 3
1 10
10 1
8 3
0 0

Output for the Sample Input

4
4

Solution in Ruby (judge verdict: Accepted)

#
# Hanafuda shuffle (AOJ id=1129)
#
n, r = gets.chomp.split.map { |s| s.to_i }
while [n,r] != [0,0]
  # p [n,r]
  
  cards = (1..n).to_a
  cards.reverse!
  
  # p cards
  
  r.times do
    _p, c = gets.chomp.split.map { |s| s.to_i}
    u = cards[0, _p-1]
    cards[0, c] = cards[_p-1, c]
    cards[c, _p-1] = u

    # p cards
  end
  puts cards[0]
  
  n, r = gets.chomp.split.map { |s| s.to_i }
end