SUCCESIVE USE OF BACKWARD SUBSTITUTION

read(a);read(b);
x:=a+b;
write(x);
{output=input1 + input2}

Step 1:
a:=input1; b:=input2;
x:=a+b;
output := x;
{output=input1 + input2}

Step 2:
a:=input1; b:=input2;
x:=a+b;
{x = input1 + input2}
output := x;