how to sum based on three different condition in R

mydata is as follow:

gcode code year   P  Q
1      101  2000  1  3
1      101  2001  2  4
1      102  2000  1  1
1      102  2001  4  5
1      102  2002  2  6
1      102  2003  6  5
1      103  1999  6  1
1      103  2000  4  2
1      103  2001  2  1
2      104  2000  1  3
2      104  2001  2  4
2      105  2001  4  5
2      105  2002  2  6
2      105  2003  6  5
2      105  2004  6  1
2      106  2000  4  2
2      106  2001  2  1

how can i get the result like this:(For example,because gcode 1 has 3 different codes 101,102 and 103,if they all have the data in the same year,and we can sum the data on that year,or we should delete the data)

gcode  year   P       Q
1      2000   1+1+4   3+1+2
1      2001   2+4+2   4+5+1
2      2001   2+4+2   4+5+1