# Variables
interval = m.FV(value=90, lb=30, ub=365)  # Maintenance interval (days)
interval.STATUS = 1  # Allow optimization

# Maintenance cost function: Periodic maintenance cost
maintenance_cost = m.Intermediate(cost_maintenance * (365 / interval))

# Failure cost function: Failures that happen if no maintenance
failures = m.Intermediate(failure_rate * (interval / 2))  # Average failures
failure_cost = m.Intermediate(cost_failure * failures)

# Total cost: Sum of maintenance cost and failure cost
total_cost = m.Var()
m.Equation(total_cost==maintenance_cost + failure_cost)

# Objective: Minimize the total cost
m.Minimize(total_cost)