试图理解：二进制表达式的无效操作数，C

Last line yields "invalid operands to binary expression". Trying to understand why. Does it mean that "p2-p1" is an invalid operand to the binary expression "-" that lies to the right of p3? Any rule I can follow here? Confusing to me because "3-2-1" integers are valid.Last line yields "invalid operands to binary ex