PROGRAM ArithmeticProgression;
VAR
a1, d, n, S: REAL;
BEGIN
WRITE('Enter the first term of the arithmetic progression: ');
READLN(a1);

WRITE('Enter the common difference of the arithmetic progression: ');
READLN(d);

WRITE('Enter the number of terms in the arithmetic progression: ');
READLN(n);

S := n/2 (2a1 + (n - 1) * d);

WRITELN('The sum of the first ', n:0:0, ' terms of the arithmetic progression is ', S:0:2);
END.