View Compilation Error

0_0_26003635_19039.cpp: In function 'int main()':
0_0_26003635_19039.cpp:446:9: error: lvalue required as left operand of assignment
    S.p.X+=S.v.X*t;
         ^
0_0_26003635_19039.cpp:447:9: error: lvalue required as left operand of assignment
    S.p.Y+=S.v.Y*t-g*t*t/2;
         ^
0_0_26003635_19039.cpp:448:9: error: lvalue required as left operand of assignment
    S.v.Y-=g*t;
         ^