river.pas

   1 var
   2   n,i,time,j:longint;
   3   last,now:array [-1001..1001] of boolean;
   4   a,b:array [0..1001] of longint;
   5   f:boolean;
   6
   7 function up(i,time:longint):boolean;
   8 begin
   9   time:=time mod (a[i]+b[i]);
  10   if (time>=1) and (time<=a[i]) then exit(true) else exit(false);
  11 end;
  12
  13 begin
  14   readln(n);
  15   for i:=1 to n do readln(a[i],b[i]);
  16   last[0]:=true;
  17   repeat
  18     f:=false;
  19     inc(time);
  20     if last[n+1] then begin writeln(time-1); close(input); close(output); halt; end;
  21     fillchar(now,sizeof(now),false);
  22     for i:=0 to n+1 do
  23       if (i=0) or (i=n+1) or (up(i,time)) then
  24         for j:=i-5 to i+5 do
  25           if (last[j]) and (j>=0) and (j<=n+1) then
  26             begin
  27               f:=true;
  28               now[i]:=true;
  29               break;
  30             end;
  31     last:=now;
  32   until (time>2520) or (not f);
  33   if not f then writeln('NO');
  34 end.