river.pas

本光盘是《国际大学生程序设计竞赛例题解（一）》的配套光盘

program River_Crossing;
const
  num='';
var
  f:text;
  n,i:word;
  t:longint;
  a,b:array[1..1005] of byte;

function up(t:integer; time:longint):boolean;
begin
  time:=time mod (a[t]+b[t]);
  if (1<=time) and (time<=a[t]) then up:=true else up:=false;
end;

function run:longint;
var
  last,now:array[0..1010] of boolean;
  time:longint;
  i,j:integer;
  std:boolean;
begin
  fillchar(last,sizeof(last),0);
  last[0]:=true;
  time:=0;
  repeat
    inc(time);
    if last[n+1]=true then begin run:=time-1;exit end;
    std:=false;
    for i:=0 to n+1 do now[i]:=false;
    for i:=0 to n+1 do if (i=0) or (i=n+1) or up(i,time) then
      for j:=i-5 to i+5 do if (j>=0) and (j<=n+1) and (last[j]=true) then
        begin
          std:=true;
          now[i]:=true;
          break;
        end;
    last:=now;
  until (time>2520) or (not std);
  run:=0;
end;

begin
  assign(f,'river'+num+'.dat');reset(f);
  readln(f,n);
  for i:=1 to n do readln(f,a[i],b[i]);
  close(f);
  assign(f,'river'+num+'.out');rewrite(f);
  t:=run;
  if t=0 then writeln(f,'NO') else writeln(f,t);
  close(f);
end.