#include #include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; typedef long long ll; typedef pair pint; typedef vector vint; typedef vector vpint; #define mp make_pair #define fi first #define se second #define all(v) (v).begin(),(v).end() #define rep(i,n) for(int i=0;i<(n);i++) #define REP(i,n) for (int i=(n)-1;i>=0;i--) #define reps(i,f,n) for(int i=(f);i<(n);i++) #define REPS(i,f,n) for (int i=(f)-1;i>=(n);i--) int dx[4] = {1, 0, 0, -1}; int dy[4] = {0, 1, -1, 0}; int main(void){ double xa, ya, xb, yb; cin >> xa >> ya; cin >> xb >> yb; double rxa, rya; rxa = -xa; rya = ya; //printf("%f %f\n", rxa, rya); //直線aB double y; double katamuki = (yb - rya) / (xb - rxa); //printf("katamuki%f\n", katamuki); //y = katamuki * (x - xb) + yb; y = (-katamuki * xb) + yb; printf("%f\n", y); return 0; }