Taxi_amount = int ( input ( "N taxi: " ) )
Taxi = []

for i in range ( 0, Taxi_amount ):
    Taxi.append ( int ( input ( "Insert city: ") ) )

TaxiBest = Taxi [ 0 ]
Prezzo = Taxi [ 0 ]

for i in range ( 1, len ( Taxi ) ):
    # TaxiBest = minimo tra ( TaxiBest + 1 ) VS ( T [ i ] )
    if  ( ( TaxiBest + 1 ) < Taxi [ i ] ):
        TaxiBest = TaxiBest + 1
    else:
        TaxiBest = Taxi [ i ]

    Prezzo = Prezzo + TaxiBest

print ( Prezzo )